1) The first thing to look at is the SAE "service level". Originally the service was to start with "S" for gasoline engines and "C" for diesel. Then the "service level" was designed to be A (light duty) through E (severe duty). So the best you could get for your motorcycle was SAE SE oils. Then they started doping oils with wear additives, so "SF" was introduced. When that market took off, they did some more and got service "SG". These days we're up to SM or SN. 

Somewhere along that time line (about SG or SH) it was decided to remove the "zinc package", which was the additive that protected camshafts but killed catalytic converters. By that time (somewhere about 1980) more car engines had "roller cam lifters" and didn't need the zinc additives, but all cars did have a catalytic converter. So the EPA decided (rightly so) that the environment was more important due to the millions of cars driving around.

All of the older engine designs (such as Airheads, lawn mowers, garden tractors, pressure washers, etc) stayed with the older "solid lifter" technology, and so these older engine designs still need the "zinc package". That reduces your choices to oils to a service designation between SE and SH. These are obviously not as numerous or easy to find, but are still available in brands like Valvoline VR1, Spectro and others. These may be labeled as "racing oils" on the front label since they are intended for cars and motorcycles without catalytic converters, but look for the SAE service label, a circular stamp (usually on the rear label), to see if it applies. The letters SF, SG, SH, etc will always be inside that circular label. It is inadvisable to use oils labeled SL, SM, SN (modern car oils) in these older or more more basic engines.

2) Next the choice of Detergent or Non-Detergent will basically boil down to what you can buy within that SF, SG, or SH service rating. Your choices probably won't be many, and so the decision will probably be made for you. 

Detergent oils are designed to pickup and carry any trash to a filter. The oil gets cleaned, and all the trash collects in the filter. If you do not have a "filter", then circulating ALL the trash in the oil ALL the time can be limiting to the engine's life.

In a car engine these filters are usually paper, but a lot of motorcycles use a slinger-style filter. I understand that the R69 uses a slinger... which means besides changing the oil regularly, "basic maintenance" will also need to include cleaning the centrifugal oil filter on a regular basis.

Non-Detergent oils on the other hand allow the trash in the oil to "settle out" by simple gravity. In those engines you will see trash start to accumulate in areas where the oil may have a very low velocity, such as the oil sump or engine oil tank. The problem with swapping the engine oil from non-detergent to a detergent style is that all the trash (which may have settled out over decades) is once again picked up and allowed to circulate. It's like drinking water from a clear stream, versus drinking water from a lake where 20 years of muck in the lake bed has been stirred up. 

3) Oil weights are mainly chosen based on ambient operating temperatures. In most of the USA, 15W50 or 20W50 will do nicely in any air-cooled motorcycle for most of the year. Weights may need to be lower for the dead of winter or higher in desert regions, but generally 20W50 is the magic number.

Hope this helps.