This is redline 75w-90 from the 4.10 LSD in my 1991 BMW 318i. The 318i doesnt use a standard BMW diff, it is slightly smaller and has slightly less capacity. The cover is finned, but I believe the oil gets very hot. BMW recommends Castrol XJ fluid (IIRC) which is a 75w-140 in all LSDs based upon a TSB. I refilled with a Specialty Formulations diff fluid and definitely found that the diff quieted a bit (these diffs are also known to be a bit whiny).
Sample Date 1/13/2008 Unit Age 111425m Comp Age 111425m Time on Oil 9925m Oil Maint. Changed Fluid Redline 75w90 Fe 188 Cr 2 Ni 1.2 Ti 0.6 Ag 0 Al 11 Pb 0.1 Cu 0.6 Sn 1.6 Si 166 Na 2.4 K 1.5 B 146 Ba 0.3 Mo 7.5 Mg 0 Ca 70 P 831 Zn 31 S 14997 Cd 0 Mn 3.7 V 0 Sb 0.2 [email protected]°C142.6 White Metal NONE Babbitt NONE Precipitate NONE Silt NONE Debris NONE Dirt NONE Appearance NORML Odor NORML H2O(Emul) NEG H2O(Free) NEG