Beemer person here, having owned 5, all 'R' series except for a K1200 LTC, so here's my humble opinion:
Since the gearbox is separate from the engine oil, you can try different weights but please don't use a synthetic oil or you may 'cause' oil leaks:surprise:
Synthetic is so 'slick' that it has a tendency, especially on older high mileage BMW's to seep past gaskets and O-rings:sad:
Believe it or not, some mechanics confronted with leaks to repair, tighten all nuts and bolts and use a 'Dyno, non synthetic' oil and the gaskets and O-rings will 'swell up,' and sometimes stop the leaks cold:grin:
Sam:grin: