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.
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'02 530i (PU 5W-40)
'15 Q5 3.0T
'13 F700 GS (BMW HP 15W-50)