I'd get Vech's recommendations, but here's what I'm doing on my R25/2 based upon my latest notes. In the engine, I use a Golden Spectro 10w40. Generally I think the manual calls for a 40W non detergent for summer and 30W for winter. The transmission has was appears to be a Castrol 40W oil...this one puzzles me as to why I chose that. Normally transmissions have gear oil in them but on general terms a 90W gear oil has the same viscosity as a 40W engine oil. In the final drive, I'm running a GL4 85W90 gear oil. I believe I got that from Vech.
Vech takes the position that a hypoid GL5 gear oil is bad for BMW transmissions and final drives. The reason is that the high Sulphur content can attack yellow metals. In his tech article, he says that before 1992, GL5 can damage the brass and bronze in the transmission. I think the story goes back to maybe some Volvo's that had this happen to them. I suppose there are varying Sulphur content in GL5 products, so maybe some are worse than others. I've run off-the-shelf GL5 gear oil in my /7 for 100K miles and haven't noticed any issues.
http://www.benchmarkworks.com/articles/tech/gearoil.html
Vech takes the position that a hypoid GL5 gear oil is bad for BMW transmissions and final drives. The reason is that the high Sulphur content can attack yellow metals. In his tech article, he says that before 1992, GL5 can damage the brass and bronze in the transmission. I think the story goes back to maybe some Volvo's that had this happen to them. I suppose there are varying Sulphur content in GL5 products, so maybe some are worse than others. I've run off-the-shelf GL5 gear oil in my /7 for 100K miles and haven't noticed any issues.
http://www.benchmarkworks.com/articles/tech/gearoil.html