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 (PPE 5W-40)
'15 Q5 3.0T (Edge 5W-40)