I can usually find valvoline synpower 75w90 at parts stores quite easily and with a lower price tag too.

I personally like to use mystik 80w90 from orielly parts or tractor supply on older differentials asking for an older spec heavy gear oil.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)