I use the Redline Dex 3 in the TC and Mobil [1] 75-90 Syn in the diffs. Switched the OE fluid out at 100k miles, at over 150k miles now with no issues.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!