Are you talking about the rear diff? In my '02, I replaced the OEM fluid with Redline 75w-90 at around 38K miles. Then I did another change at 70K miles with Lubro Moly 75w-90. Haven't really noticed any difference with any of them.
'02 530i (PU 5W-40)
'08 C300 4Matic (M1 0W-40)
'13 F700 GS (BMW HP 15W-50)