Helping my neighbor this weekend with his 2006 C350. One thing on the list is rear diff gear oil change. Will the world stop spinning and the rear end drop out if we use Castrol Syntrax LS 75W-90 which he has in his stash or do we need to get a spec'd 235.7 oil which would mean from the dealership today? I see the 235.7 is 75W-85, is this for better fuel economy?
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