Best oil for my airhead

[Wern2be]

Hallo guys

Ive been on a few sites and the most guys like to use 80w/90 oil, but as far as my knowledge go this is a very thick oil...what oil should I use.

BMW R60/7
BMW R100RT
BMW R100RS

Thank you Werner

[barryh]

80W90 GL5 is the correct oil for year round use in the gearbox, drive shaft and final drive. If you want something thinner it would have to be a semi synthetic or synthetic 75W90.

[Rebel]

I presume you are speaking of final drive and not engine oil which should be 20W-50?

[barryh]

Although we tend to think of gear oil as being thicker than engine oil, 80W90 is actually similar in viscosity to 20W50. In fact 20W50 is a little thicker and the published figures will bear that out. At 100 Deg C 80W90 is 14 cSt or much the same viscosity as 10W40 engine oil while 20W50 is thicker at 19 cSt. The SAE engine oil scale and gear oil scale are different which explains the anomaly.

You still wouldn't want to use 80W90 in the engine but it explains why some gearboxes do run on engine oil not least many motorcycle gearboxes that share the engine oil.

[Dan in IL]

If you have any issues with shifting when the gearbox gets warmed up, use something like 80w140. It made a definite improvement on my ol' R75.

[Roy Gavin]

A G/L 5/6 oil is one step up from a GL 5 and I have found an improvement in shifting using it.
Most oils I have seen for limited slip diffs are 5/6 so it is not too hard to find.