I am pretty sure it calls for standard 75-90 GL-5 gear oil. I would run a full synthetic. I have used Amsoil, Redline and Royal Purple as well as Valvoline for rear end applications. For driveline fluids I like Redline best.
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5