If you haven't changed the seals in the final drive and transmission, then 30 weight engine oil is recommended. If you have changed the seals in the last 40 years, then you can use GL-4 or, perhaps, GL-5 gear lube. There is a certain amount of controversy about whether the formulation of GL-5 will attack the "yellow metal" -- brass or bronze -- parts.
For the motor, there is endless controversy about whether you should use a) straight weight low or non detergent oils (30 weight summer, 20 weight winter); or b) modern multiweight (20W50) motorcycle or SG rated auto oil; or c) any modern 20W50 oil.
Having laid out the possibilities, I will let others argue for their particular choices.
The BMW oil is rated at SG. This rating ensures that it still have a fair amount of ZDDP, a zinc-phosphate combination which provides good anti-friction qualities. However, a seem to recall an issue with our engines having roller bearings as opposed to the later bearings which do more sliding. I haven't quite figured that one out yet...maybe someone can enlighten us.
Use a quality oil and change it often...that's what it comes down to.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2