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Front Fork Oil discussion; filling amounts; hints.
This article applies to Pre-Airheads, Airheads, Classic K bikes

http://bmwmotorcycletech.info/front-fork-oils-amounts.htm
section 54-10B
© Copyright 2018, R. Fleischer

Discussion of Oil types and characteristics.

The following article SHOULD BE USED WITH: http://bmwmotorcycletech.info/viscosity.htm. That article has information that REALLY SHOULD BE READ...either NOW, or, after reading the below article!

I prefer Spectro's fork oils and suspension fluids. They are of good quality, have low stiction, wide temperature range (excellent viscosity index), & the viscosities can be depended upon. Due to how specified, & lack of stiction fighters in some oils used commonly in our front forks & other characteristics, you are better off with a real fork oil like Spectro's ....especially the full synthetic or part synthetic fork oil. For fork oils & suspension fluids, the various manufacturer's do not agree on measuring viscosity; sometimes they don't do more than give some sort of approximate SAE grade value. Viscosity measurements & temperatures are vastly more accurate between manufacturer's for engine & gear oils; not so, apparently, for many suspension & fork oils. Because of these various things, and other reasons I won't bother to get into, I highly recommend you stay with one manufacturer, this is particularly so if you are trying different viscosity grades.

I have not yet done extremely long period testing to find out what oils, or ingredients (esp. mineral vs synthetic), are causing the deterioration of the 'bumpers' at the bottom of the forks.  Until someone does such a test, or an accelerated test (increasing the temperature?), it is best to simply change the fork oil at reasonable intervals. 

Back in early Airhead & K bikes days, the oil BMW used was "BMW red fork oil". That oill at least in the USA, was actually a military hydraulic oil, you could find its full specifications using a search engine for: MIL-H-5606 (with whatever the latest letter suffix is). Oil with a "5606" military specification can be depended-upon for viscosity (but not for some things you really want for your fork oil). To save you the trouble of finding the information, just the viscosity specifications for that oil are:
MINimum 4.9 Cst at 100° C (generally accepted as 210°F).
MINimum 13.2 Cst at 40° C (100°F).
Maximum 600 Cst at -40° C ....too cold to bother about for motorcyclists (unless very crazy!).
You can use those two viscosities, 4.9 Cst and 13.2 Cst, to compare (perhaps!), with other manufacturer's oils specs ...& with the information in the following article on this site:
http://bmwmotorcycletech.info/viscosity.htm

DO NOT DEPEND ON MANUFACTURER'S SAE NUMBERS ...USE MY CHARTED INFORMATION in that above LINK, for oil viscosity information!

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