I used Mobil 1 and swore by it, but after I got the E36, M1s weren't the best for it (I used 0w-40 during the winter) so I switched to Castrol Syntec (OEM Synthetic Oil for BMWs) 10w-40 in the summer. The car seem to run smoother with Castrol so I'm sticking with those. Currently I'm running Syntec 5w-30 for the winter (7 quarts!).

I'll never use Mobil 1 for tranny or diff fluid. Their protection blows, so I'm gonna try Red Line.