I recently did both the front and rear diffs with Valvoline Syntec 75w-90. To my demise, the rear had a leak in the front input shaft seal. New fluid, flowing better - went right out in 2 days and I burned up my diff on the highway.

Just a note - make sure you're not leaking any fluids right now before you replace it or you could accelerate the damage.

Eventually, I had to get a used diff. My '06 has 90K on it but the used rear diff has 18K. The indy I got everything replaced at used the original Castrol XAF-XJ fluid. Check with the dealer and confirm with another as it may be different for the models with and without X-drive.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean