I’m not sure what is meant by semi-synthetic, but for the final drive my Haynes manual calls for Castrol synthetic SAF-XO (apparently re-branded as Syntrax). I’m not a believer that the brand of oil makes much difference so I did a little bit of research, confirmed that 75w-90 and GL-5 are the required weight and standard and went with Mobile-1 synthetic 75w-90, as this was available at my local auto parts store.

I have not changed the gearbox oil, but in my research I saw a lot of suggestions that using 75w-140 helped smooth out shifting. However, the Haynes manual also calls for Castrol synthetic SAF-XO, or ‘API grade GL5 SAE90’

Wade

 

Yep! I use Castrol Syntrax 75w90 Longlife (synthetic) for FD & transmission, which I get from my local BMW dealer.

 

BMW recommends Castrol SAF-XO (now Syntrax) but Like Wade says, the viscosity and standard are the key specifications - 75W-90 and GL5. BMW recommends Castrol, but any brand meeting that viscosity and spec will do the job.

I can't comment on going to a heavier weight, except to say that if it was beneficial, wouldn't BMW have specified it? Maybe a lighter oil is better for fuel economy, whereas a heavier oil better for gear changes, and BMW has made their choice on this trade-off - or they've simply stuck with their preferred supplier's product ...

Anyway, you can't go wrong with Syntrax or its equivalent.

 

I asked about both oils on bobistheoilguy.com, the motorcycle (and car) related site with the highest concentration of oil experts.

https://bobistheoilguy.com/forums/ubbthreads.php/topics/4180815/Re:_BMW_Hexhead_gearbox_oil?_R
https://bobistheoilguy.com/forums/ubbthreads.php/topics/4164755/BMW_final_drive_75W90_GL5._Sup

As a result of those I went with Redline Shockproof Heavy in the gearbox the two times I've changed that, and been very pleased with it. I went with Supertech 75W90 in the final drive. It's cheap, meets specs, and seems to work.