Revised 01-26-2021

 

This discussion is also applicable to most old flat tappet engines, including some VW, Porsche, old Corvettes, etc.

Over the years, oils have been a popular subject for discussions in our motorcycle publications, club magazines, & on-line LISTS & Forums, Facebook, etc. I am not sure just why that is, but I suspect it has to do with our society of easy fixitus, or ‘I need to do something to feel good’, maybe a panacea for the masses…and maybe male ego, ..and who knows what else!  Certainly there is a large amount of placebo effect. REAL facts are hard to come by, and so-called ‘evidence’ abounds, and may be quite difficult to verify. There is a considerable amount of so-called ‘information’. Some is good, some is anecdotal, some based on testimonials, some sort of like ‘my brother told me of his friend who has a friend who told him that…’; or, ‘my mechanic who has decades of experience, says….

Did you ASK that person just HOW HE REALLY GOT that “information” he is giving you?   Does that mechanic friend have a sweetheart ‘deal’ with a particular oil’s distributor?  Has he ever REALLY seriously tested oils in Airheads?   How did he test them?   Has he looked at hundreds of Airhead cams and lifters?   Do YOU believe the stories often heard>>that “car oils are plenty good enough, you hardly need pricey specialty oils?”.    Some information even seems scientifically derived, and implies itself to be definitive; but, in MANY instances this is NOT SO.   It wasn’t all that many years ago that we saw a lot of mass advertising about engine oils or engine oil additives.   There is much less of that NOW, MAINLY BECAUSE FEWER AND FEWER FOLKS ARE CHANGING THEIR OWN OIL IN THEIR CARS (AND CAR ITEM SALES DRIVE THE OIL MARKETING, NOT MOTORCYCLES).

SORRY, but those hyped additives like Slick50, QMI, etc., and those tests on TV of engines run without oil….these are sideshows; you never hear the entire story of the tests.  DO NOT believe these folks. YES, it is true that you MAY (or may NOT) have SLIGHTLY less friction, SLIGHTLY higher fuel economy, etc. HOWEVER, you will also likely get much increased WEAR (YES, you can have LESS friction and considerably MORE wear!), the product may settle out or ‘clump’, thereby plugging oil passages and oil filters, ….and LOTS more ills. This can lead to oil starvation and catastrophic engine damage. As for old-time additives like Bardahl or Rislone or Marvel Mystery Oil: don’t use them, they can remove the protective coatings on your engine parts.
BUT:  There ARE A FEW places that Rislone can be of help, and one is temporary use in a Classic K-bike, to hopefully free up a slipping starter drive clutch….aka Sprag Clutch.

Motorcyclists are a smallish group of folks that end to do a lot of their own work.  That includes oil changes.  But, the total market for motorcycle oil is very small, compared to car oils.  You also will not find places that you drive your bike into, and have a ‘quick oil change’.   The marketing for motorcycle items, including specialty oils, is limited.  Thus, with modern Media, it is easier for rumors, untruths, etc., to gain a presence.   As motorcycles become more and more likely to have such as catalytic converters (which don’t like ZDDP in the engine oil), and the market for specialty oils narrows, with occasional blips, it is easy for bad information to get distributed.

OK, have I got your attention yet?   Have I offended everyone?

In preparing this article on oils I not only relied on MY background experience, but that of several very well known and RECOGNIZED experts on Airheads.  I even had direct access to two ‘real’ experts, one an engineer with Chevron, and the other an engineer who specialized in lubrication, and who had worked for two car engine manufacturer’s and had been a consultant with a motorcycle engine maker.  I also did a fair amount of research, including tests on pour temperatures, slipperiness, and shearing …mostly just for my own information; but also, to ‘put holes’ into advertising hype. I made up some jigs & tests to do this. I spoke to some other petroleum engineers and obtained some off-the-record information. I looked into some specifications not normally available, on additive types, actual temperature effects on residues, neutralization number (a measuring method and answer dealing with how well the compounded oil neutralizes acids), channeling (when an oil is so thick at a low temperature that it does not flow at the gear meshing surfaces, and so the metal is starved for lubrication), shear strength, film strength, shear resistance of the VI improvers (additives), load carrying, corrosion resistance, foaming problems, detergency, oxidation resistance (oil, particularly petroleum, will oxidize at higher temperatures and gets thick and flows poorly).

I also dug rather deeply into testing done by various biker magazines, and found a lot of fault with these tests, which can be extensive and appear to be definitive and are NOT.  Motorcycle Consumer News, long gone now, did a very decent set of testing (more than once) on many oils, and while one of the best, if not THE best reporting, it was very faulty in regards to AIRHEADS, which have flat tappets and special considerations.

…AND, lastly, I looked into the one item that I could NOT get 100% straight & definitive answers on what I wanted to know about:  seal compatibility; although I did get some information about the seal compounds used.

I paid a considerable amount of attention to lubrication effects and the effect on the lubricants by the high temperatures found in our Airhead valve areas. We do not have starters or clutches in our Airheads oil, and we need good shearability protection in the transmission, which is why BMW wants us to use GL5 gear oils there…not just in the rear drive.    PLEASE disregard ‘old stories’ about GL5 gear oils being bad for Airheads gearboxes or rear drives, GL5 supposedly eating brass parts….  IT IS NOT TRUE for Airheads, and only partially true for PRE-Airheads, details and specifics by model and serial number are in my own website.

A bit of history:  BMW’s own labeled oil was manufactured, up until early 2013, by Spectro, and WAS NOT Spectro 4 or Golden Spectro 4.  This does not mean much, …and that particular BMW oil, while “OK” for our Airheads, was really designed for the Classic K series (the early K models, like the K75, K100, K1100), in which temperatures are vastly better controlled, being water cooled motors…AND, except for the first year or two of Classic K bike production, quality car oils, THEN, were mostly OK for them.   Strictly speaking, the BMW labeled oil was not as good for Airheads as it could be, but was acceptable.  REAL Golden Spectro 4 was BETTER, particularly for Airheads.

I am not going to go into some of the more esoteric oil terminology, like base stocks, polymers, bright stocks, uptreats, synthetic oil bases, diesters, polyolesters, polyalphaolefins, alkylated benzene’s, etc., and all–or most–of the terminology used by petroleum geeks.

What owners of Airheads should be concerned with are the basics, and that some motorcycle oil manufacturers additive packages are quite good, over-all.   They tend to activate at AIRHEAD temperatures, do not get burned up quickly; that is, they are not overly volatile; and, many if not most have a decent amount of a proper, for Airheads, ZDP/ZDDP (a zinc/phosphorus compound). That compound is an extreme-pressure, anti-wear compound.   IT REALLY DOES NOT DO VERY MUCH UNTIL THINGS APPROACH OR GET TO EXTREME, LIKE METAL-TO-METAL CONTACT, OR NEARLY, WHICH  IS NOT A CONSTANTLY NORMAL SITUATION, BUT CAN AND DOES HAPPEN, often.  ZDDP is also the LAST LINE OF DEFENSE, PARTICULARLY if you like to go up to or near redline rpms, which causes excessive wear on our engines. Going close to yellowline, let alone redline rpm on a BMW Airhead is of no benefit, particularly in performance; &, I believe, is hard on nearly everything, including even alternator rotors when you begin shifting down (reverse torque on the rotor). High rpm’s near red line wears things out much faster.   It tends to scrape oil films off parts too. That’s not exactly correct, but good enough for here.   That does not mean that you should ‘lug’ the engine by riding at 2500 with plenty of throttle! Our Airheads perform really well between about 3800 and about 6000. You will not injure the engine by cruising for hours on end at 4500-5500 or so.  It is when you get within maybe of 1000 of redline …and higher, …that wear increases so much faster.  Using throttle too heavily at low rpm, especially with a cold engine, is not a good idea.  Zinc and phosphorus compounds also protect some metal surfaces under conditions of high heat, high rev’s, racing, etc. The primary protected surfaces are cams, lifters, cylinder bores, & piston rings. Once the necessary amount of the correct version of the zinc compound is present, you get no more protection from more.  Excessive zinc can increase deposits on your valves, foul spark plugs, etc. Only for race engines should a super-high ZDDP oil be used, and only for break-in, and these are expensive specialty, special order oils.  Forget about them!

Zinc has been reduced in the later oils (SJ, etc.), and eliminated in CAR oils for the most part in the latest xx letters oils because zinc damages catalytic converters in cars. It only takes a very tiny amount of the zinc to give the proper protective effect.  I am in agreement that about 0.11% is about correct for good-enough protection, and I also think that .20% is likely pushing the idea, and probably much more than needed  …or desirable.   For just the ZDDP reduction reason alone, car oils should not be used in our motorcycles.

I will mention, since the subject does come up now and then, that years ago use of straight weights of oil were recommended, particularly for severe service, in that the multigrade oils then available were not good. This was in the days when the thickeners (OK, polymers) were not all that shear stable. What happened was that the moving parts tended to ‘shear’ the oil, resulting in a lowering of the higher number (the 50 in 20W50, for instance). For quality oils, the extent of the shearing problem has lessened considerably.   A complicating factor, way back then, was that multigrade oils, of any quality, were not common in poorer countries (use your own words…).

There are some FEW who still use single grade oils in BMW’s. Usually these folks live in extremely hot desert areas.  Single grade oils generally are burned up through the valve guides, and multigrade oils tend to reduce consumption at the rings…..are you REALLY concerned about adding a half quart in 3000 miles or whatever? ….. what about long-term wear?

I WILL FLATLY STATE THE FOLLOWING: QUALITY multigrade oils are better in ALL respects for Airheads… AND.. PLEASE…do not add any additives to your oils (except that Rislone mentioned additive for K bikes, temporarily only, if needed, for sprag clutch slippage).

If you often start your BMW below perhaps 20°F (there are SOME who do! BRRRRH!), then it might be better for you to use a full synthetic oil; or, use a 10W40 semi-synthetic oil (which I recommend if starting at freezing or below).  If you intend to use a full synthetic oil at all, do not install it until your engine has broken in…probably a couple thousand miles.  By then the rings have very likely broken in properly and oil consumption has stabilized and hopefully quite low.   That recommendation has not changed over the years. If your engine is getting tired and is burning oil, you may not want to use synthetic oils, although there may be no actual problem, just usage of a pricey oil.  WORN engines actually require BETTER oils, otherwise they wear QUICKER, and even more quickly as the wear continues.    So; I recommend a semi-synthetic oil, as the best compromise.   Golden Spectro 4 has been a very steady recommendation by me, and some others I trust, for REAL reasons…..including inspection of a whole lot of torn-down engines.

It is oil degradation that is the MAIN reason that petroleum oils need to be changed regularly, and NOT contaminants from combustion products. This was not always 100% so, back in the days of heavily leaded gasolines.  Thus the longer drain intervals of semi and full synthetics is likely justified…but NOT over 10 or 12000 miles in one season, and those figures are for ideal conditions.

Your BMW oil filter is not designed to be a ‘depth filter’, the word ‘depth’ being a code word for long mileage/long time.  However, it is almost always OK to change the Airhead oil filter at every other normal oil change interval. The filters are typically changed at about 500 or 600 miles on a new or rebuilt engine, and ALL removed filters, every time, should be taken apart and inspected on both sides of the filtering medium, a type of treated and pleated paper.   The HINGED BMW filters are stronger and better; and SOME aftermarket filters filter poorly….are NOT good and even not safe to use. If a filter should collapse, you could have $$$ problems.

Most engine wear occurs at the first minute or two from startup from cold…that is just one reason a multigrade is better….AND, many multigrade oils are also better than single grade oils at the high end of temperatures.

Oils vary CONSIDERABLY in quality, and it is NOT just a matter of the additives. It is the TYPE of petroleum, the quality and combination of the additives, etc. Some cheap oils will burn off rapidly, and/or their additives will burn off or evaporate off (the usual mechanism) rapidly.  That means you may loose the first half quart or quart rather rapidly.  Some oils have synthetics added to help avoid this, or to modify other straight petroleum characteristics.

Thus, except for special instances, a semi-synthetic might be the best choice of oils for your Airhead.  One of the best semi-syn oils is Golden Spectro 4.   Most airhead owners in mild climates use it in 20W50 grade, and 10W40 in Winter climates.

The price of oil, by the single quart (or liter) varies greatly.  The extra money often does not buy you anything but perhaps peace of mind.  There are Diesel engine oils that are, or were, very good, because they contain ZDDP together with other and better oil formulations.  Oils like Delo and Rotella, in standard AND synthetic versions, are, or were, good oils.  Many an Airhead owner who does not want to pay the $$ for a motorcycle-specific oil, will use Delo or Rotella.  You can’t use ZDDP-containing Delo or Rotella in a gasoline engine car with a catalytic converter.   These car/truck oils manufacturers have been reducing ZDDP content more and more as the years go by.  Finding out the ppm of ZDDP in oils is often frustrating…and may change without notice by the manufacturer.  900ppm of ZDDP is adequate, if you can find out, and 1300 to 1600 is very good.  Spectro used 1800 in most of its oil, and it is about the maximum I think to be considered.

If you REALLY intend to keep your Airhead for 100K++, then use a quality m/c oil designed-for and sold-for air-cooled motorcycle use…and if you insist on a cheaper oil, use Delo or Rotella.   There ARE reasons to use quality motorcycle oils…..ZDDP; good base stocks, good additives….and YOUR Airhead might be the one with the marginal factory heat treatment on the cam and cam followers…riiight?

Regarding CX oils: I don’t want to get into this in depth, BUT, oils GENERALLY come rated on the plastic bottle/can as having BOTH a CX and SX rating. These oils are OFTEN, but not always, a COMPROMISE. There ARE some very good older oils that were rated both SG and CD. Keep in mind that the ZDDP additive we want so much for our cam faces, lifters, and cylinders/rings, come in different grades, costs vary for those, and certain grades work better at higher temperatures and pressures. Some have a wider RANGE of operating temperature. Of course ZDDP is not the only difference in additive for CX oils. CX graded products are manufactured to release usable additive at a higher temperatures found in diesel engines, and this is not a good idea for Airheads, unless they release at lower temperatures also.  Something not-at-all-talked-about is that these temperatures refer only in passing to the oil temperature in the crankcase; rather, they refer to the temperature of the oil in places like the rod bearings, and so on, where oil flow (volume) does the COOLING from the friction, and the VERY major part of the friction is from the thin oil film layers rubbing on themselves!  Some modern MOTORCYCLE oils may be rated as JASO MA and MA2.   NEITHER means the oil is truly what you may think.  MA simply meant that the oil was OK for wet clutches and had a bit of protectant against metal deterioration in such as gearbox transmissions. MA2 was a later specification, and it was to be OK for use with catalytic converters.    As you probably have guessed, it may not have any or enough ZDDP to help your Airhead engine, no matter what else the container says!

Golden Spectro 4 has a very good additive package, including some synthetics, & is what I personally use in my AIRHEADS,,,,but I am NOT pushing this on you, there ARE others, some are cheaper too. I’d be using Delo or Rotella, otherwise…until they reduce the ZDDP under 800 ppm or so.  I would not be a bit surprised if my use of that Golden Spectro 4 oil was not a bit of an overkill, especially since I change it regularly, and I avoid yellowline and of course I avoid redline. I also think that, for many Airhead riders, the cheapest oil (even an SJ) will be adequate. However, here is something hardly ever discussed, and probably the root cause for those anecdotal reports of vast mileages on the cheapest oils:
HEAT TREATMENT of various metal parts may vary considerably between any two or more Airheads. Particularly in cam lobes and lifters (followers).

You should change the oil regularly, even if it is synthetic or has some synthetics. That is MY OWN belief; backed up by sending oil to laboratories for analysis, something I got into when I purchased my first airplane, where such analysis really pays off. I continued such testing with some of my Airheads.   Change intervals depend on your riding. You all know this stuff: if you do mostly stop and go in town traffic, or pull a trailer or have a sidecar, or ride short under 10 mile trips where the oil never gets hot enough to burn off the moisture accumulated; or ride in very dusty/dirty areas;  change the oil every 3000 miles or so,,,,otherwise 5000 or even 8000+ is likely acceptable with part or full synthetics, and mostly high speed highway miles. Change the filter every other oil change, inspect it carefully for metallic looking or plastic looking particles.

Here is what I believe about Mobil 1 full synthetic:  This oil, forget the cap color controversy, is often found considerably cheaper than fancy motorcycle synthetic oils, and Mobil 1 is available in a desirable
15W50 grade. Sometimes even Wal-mart has large sized jugs (5 qts?) on sale. I can add a full synthetic Delo or full synthetic Rotella-T to the discussion here.  I believe these oils can be left in an Airhead engine for up to maybe even 2 years, if mileage is under 12,000; and stop and go riding or especially very short trips (say under 20 miles) is at a minimum.  A further requirement would be that not long after a lot of stop and go in slow city riding, one at least does a ride over 20 miles.  I have NOT proven this totally, but it IS indicated in laboratory tests.   Mobil also makes a synthetic 20W50, etc., called V-Twin oil.  It has high ZDDP, and is a quite good oil, in my opinion.  Mobil 1 is likely THE oil to use, not on Airheads (you can, however), but for Classic K bikes, which have a tendency to have problems with the Starter System Sprag Clutch.  I’ve never heard of a problem with that mechanical monster part, and the owner consistently used Mobil 1.  I used it exclusively on my K bike too.

When I originally updated this section of this article in November 2007, I was doing very long term tests (at low mileages) on Golden Spectro 4, 20W50.  This was on one of my own Airheads, and I intended to stretch the oil change to maybe 4 years, but under 5000 total miles (yes, not a misprint).  I finished that test.  The results are incorporated into this article.  I rechecked and updated in May 2017…yes, 10 years later.

Contaminates accumulate in oil between oil changes, no matter what type of oil you use, including synthetics. Some of these oil additives, used by the oil manufacturers, wear out, and are only replaced when you change the oil. There really is a limit to the change interval. JUST what that limit is for each oil is not the type of thing you can easily find out. For sure the manufacturer’s will never tell you!  The contaminates accumulate fast at cold startup.  Petroleum base oils have sulfurs, some more, some less.  They are much worse at this sort of thing than most synthetics.  At startup, moisture  accumulates in the oil, and tends to create acids with the sulfur. Acids EAT parts. This is only ONE OF SEVERAL REASONS NOT TO run your engine in your garage for ‘a few’ minutes every so often during Winter storage!! If you store your bike in the Winter, or anytime really; change the oil and filter and run the fresh stuff through the engine (cranking, no starting) before storage, and do not start the engine until riding season. Use a good oil.  A cheap oil may NOT leave the parts coated with oil during long storage.

Synthetics: I used to not believe the much greater cost of a 100% synthetic would pay for itself in anything but very cold climates;…even considering the hype over longer change intervals.  However, synthetics are looking better and better for most everyone; this is especially so with the cheaper synthetic diesel oils like Delo and Rotella and of course full synthetics like Mobil 1.  A good synthetic may well burn less, provide easier starting, and stay cleaner longer.  It may well not cost too much more, or, may even pay, over the long run.

Does your synthetic oil have an additive to control seal swelling? Sort of a misnomer! Did you know that without that additive in many types of synthetic oils, that your seals will likely SHRINK and leak? SOME seal treatment chemicals are lousy!…at least on SOME Airheads.   That does NOT mean that I am against full synthetic oils; my feelings about them has mellowed over the years as they improved, and I know that ones from Spectro and even Rotella syn and Mobil are quite good.    Another thing not known is that some very old seal materials were not as compatible with modern synthetic oils.   The BMW Airhead main seal MIGHT have been a case in point….and that seal was changed a number of times by BMW over the years.  The latest weird-looking Teflon seal seems compatible with any type of oil.

Oils in general: Your oil must have the proper additives to neutralize the acids your engine produces from gasoline and water from the air. Sulfur compounds (especially in petroleum oils) and water are hell on engine bearings, which get eaten away, sludge forms, the corrosive products get burned up in the oil and tend to form tough varnishes, cause sticky rings, etc. You may well see something in your owners book about oil for your airhead, specifying SE oils. There were lots of problems with garbage SE oils produced by some manufacturers. It was because of these borderline SE oils that SF came about. This was somewhat before the engine manufacturers started specifying ‘new’ oils that supposedly had less friction.  BMW airheads were designed for a GOOD SE oil, and they LIKE SG oils. They might NOT be too happy with anything past SG. THE problem is that the SG rating gets mightily confused in advertising and how the cans/bottles are marked, and frankly, especially how marketed.  Here is something that is a bit subtle, that may have confused you previously:   Many “”motorcycle-specific”” oils are sold as SG; or, are sold as ‘meeting the quality specifications of SK”…or somesuch words, perhaps the SK is SJ, or after SK..SM, or, whatever later letters.  These OILS are, however, NOT REALLY LICENSED to REALLY be SJ, SH, SK, etc.  What they ARE is SG (perhaps), containing ZDDP; which is fine and dandy for your Airhead!…but would POISON car catalytic converters.  These oils can NOT contain some additives, like ZDDP in goodly amounts and STILL meet car and other specifications for SJ, SH, SK, ETC.  In a FEW instances, the oils are not ‘energy saving’ enough, and so on.   It is a subtle point here that I am trying to get across.   If the (non-motorcycle specialty) oil is officially marked as a SJ, SH, SK, and so on, and NOT SG, do not purchase it…it is for cars, NOT YOUR AIRHEAD.   If it has that wishy-washy “meets quality standards of…some later Sx” type of thing, it may well BE what you want if it is being sold very specifically for motorcycles.  Read carefully.  If you have to, look at the manufacturer’s website information…..carefully. Dig, ask, you may learn something!  Remember: the speciality manufacturer’s of motorcycle oils may have markings that say “meets the quality standards for…”, and yet actually be an oil that is NOT for cars, and IS what YOU want for your bike.  This is not easy to understand….and the manufacturer’s websites are misleading, on purpose!…so they can sell to car owners, or, well, not really sure why/what.  Made to a quality is not at all the same as MADE TO BE.  This bit of confusion came about partly because of the desire to SELL oils, and partly because of how SAE and others in the automotive industry wanted car oils to work as best possible FOR CARS….and, to Hell about motorcyclists.   Harsh words, amazingly true though.

Techy stuff:

Viscosity index is a number that indicates the RATE of change of the thickness of something, within a given temperature range. A HIGH index means the thickness varies LESS with temperature changes. ENGINE oils are specified at 210 degrees Fahrenheit or 100 degrees Centigrade for foreign oils, except that engine oils rated W are specified at 0 F (-17.8C). This means that a 20W-50 is specified at TWO temperatures, one for 20W (at O F) and one for 50 (at 210 F). If an oil does not have a W (at one time there were single weight oils with W and multigrade without W) then it is not specified at O F. For the REALLY nerdy, SAE ratings are measured or specified in centipoises at 0 F and centistokes at 210 F….and let’s not get into Saybolt Universal! Converting between these is a mathematical game.
This link has several extensive articles on oils, and how they REALLY work in the engine: https://bmwmotorcycletech.info/index.html.  There is even a separate article on real testing of viscosities for a number of oils, including engine, gearbox, forks, etc.

CX oils have specifications for sulfur content, temperatures for release of various additives, etc.    CX oils are now up to and past CF, and oils are quite common in the auto parts stores now that are rated SJ/CF (OR BEYOND). I even, quite some time ago, saw a container of Mobil 1, a full synthetic, and all grades said SJ/SH/CF. A bit later, same model said SM/CF and had added several European car type ratings.  This changes altogether too often! The important thing about CX oils in general is the protection for bearing corrosion and high temperature deposits that they were designed for, and THAT THE THERMAL STABILITY OR BREAKDOWN POINT OF THE ZDDP TYPE ADDITIVE MUST MATCH THE OPERATING TEMPERATURE  OF THE ENGINE OIL PAN AND IN THE BEARINGS UNDER DYNAMIC CONDITIONS.  You will NOT find that information out easily.    NO ZDDP?….I don’t really want one of those oils in my Airheads.
I’d rather use a ZDDP-containing diesel oil.  Delo or Rotella anyone??
It was fairly critical years ago that CC oils not be used in diesels that ran hotter and needed CD oils. At one time, we had to watch out for oils rated SE/CD, this was because those oils favored high temperatures of turbocharged diesels, and the additives did not work well at lower temperatures of gasoline engines. Another interesting point is that diesel engines often run on very high sulfur-containing fuels. As you all know, some fuel and combustion byproducts get into the crankcase oil, contaminating it, so oils are compounded to deal with the type of contamination expected.  If you get the idea here that good diesel oils DESIGNED for long-haul, high mileage between oil changes, contain a big package to combat acids and other fun stuff, yes, you are correct.  Both CX and SX oils (the X here meaning whatever letter is in use) have been changed for many reasons over the years: antioxidants, pour point depressants, anti-foaming additives, anti-scuff additives, detergents, slipperiness, other anti-wear additives, etc.   In general, diesel oils designed and sold for JUST diesels are much better for an Airhead engine than a SK car oil, and so on.   I recommend against the cheapest diesel oils….but the long haul truckers type of oils…like Rotella…that is another story.  Good oils.  At least until the ZDDP falls under 800 ppm…because the manufacturers are tinkering with that.

GEAR oils are specified for different temperature ranges than motor crankcase oils, this is particularly so with the W grades of GEAR oils. Because of this, gear oils of maybe SAE 75 or 80 are similar in thickness (viscosity) to an engine oil of about SAE 40.    Use GL5 oils for your gearbox, driveshaft (if you have a wet type) and rear drive.  I’d rather see you use a 75W-xx, than a 85W-xx.   There can be some good reasons, like hauling a trailer or sidecar with your AIRHEAD, that might make a wider range …up to 75 or 85 by 140 or 145 type oil nice for the rear drive, but in general it is not a good idea, as the typical oil viscosity on a typical day and typical riding will be too thick….and I would NOT use it in a late BMW single sided drive …VERY particularly the BMW drives with the HOLE through the center of the rear drive (where an axle usually went).  This hole appears only on very late model BMW bikes.  The best oil for most Airhead riders, for transmission, driveshaft, and rear drive, is 75W90 (cool/cold weather) or 80W90 (summer or mild weather) in GL5 type…NOT GL4….just like BMW specifies!

FORK OILS:

BMW red fork oil, back when Airheads were being sold in the USA, was basically a military Mil-5606 hydraulic oil.   For forks, you want an oil with a rather high viscosity index, perhaps over 300, as you don’t want your suspension to act differently as the temperature changes. Modern fork and suspension fluids are MUCH better than Mil-5606.   There can be a big difference between one manufacturer’s 3, 4, 5,  7-1/2, 10, or 15 wt oil, and another manufacturer IN THE SAME VISCOSITY RATING!!! Fork oil viscosity ratings are NOT government controlled!  If experimenting with oil viscosity grades, use the SAME manufacturer!  Oil level in the forks should be fairly equal (some later forks have UNequal amounts and even a different viscosity, left to right) and the proper amount.   Only a small increase in amount is useful in SOME situations.   Don’t use excessively heavy fork oils, they are NOT a good substitute for properly operating forks.   BMW has changed the amount of oil used in all their forks several times over the years.   Oil quantity in some books may be either dry (just overhauled) or wet (drained to drip free). If possible, try to get a fork oil that the manufacturer states has stiction minimizing additives. Oils that are marked Suspension fluid or oil and those marked Fork Oil, are often interchangeable in usage in our forks. Be careful not to get an oil that has too high a viscosity. Although the internal design of BMW forks has changed considerably over the years, it is still true that going to or over 10 wt., except for those BMW’s specified for 10 or 15, and except for modified forks in racing, will give an uncomfortable, harsh, overly stiff ride, and you will not like the handling, particularly over tar snakes and other small irregularities in the road.  Thick fork oils are NOT a good substitute for sacked-out springs or high stiction from misalignment!

FRICTION:

Friction is a difficult subject, there are many types. Oil separates the potentially rubbing surfaces by very complex means. An ideal oil is NOT one that would not change viscosity with temperature changes. High pressure alone increases the viscosity of an oil [surprisingly], and this is why a thin oil can support heavy loads, like in the rod bearings, but high viscosity increases fluid friction.

Rotational surface speed of moving parts influences the heat developed, and high rotational speeds mean faster shearing of oil itself, but speed helps form a cushion in bearings, so higher rotional speeds generally allow higher loading…that is ONE of the reasons why lugging your engine is BAD.

SO, what should an owner of a BMW Airhead do for engine oil? To my way of thinking, an owner would BEST use a quality oil, designed for motorcycles, in 15W50 or 20W50 grade (unless you are riding in below freezing weather, then use 10W40), and the compounding should probably be part or full synthetic, and the zinc package should PROBABLY be at least 0.10%, but not over .18%, and you will have to depend on the manufacturer’s reputation in the matter of the quality of the base stocks and the zinc package, etc.

Don’t believe all the hype.

I don’t have any problem with those of you wanting to use synthetics in the engine, transmission, driveshaft, and rear drive. SOME will not be compatible with seals.  If yours start to weep/leak,change back to a dino oil, and allow a few weeks for it to swell the seal so it does not leak.

NOTE!!….this article is not necessarily always kept totally up-to-date.  See the author’s website for a more up-to-date version:
https://bmwmotorcycletech.info/index.html
snowbum
ABC 1843

Revised 01/26/2021