If you have a auburn differential shy away from synthtics, they do not recommend its use. Any brand name 80w-90 gear oil will work fine. If you can find chevron 80w-90LS or mobil 80w-90LS, or white bottle Valvoline, you will not/should not need to add the friction modifier, maybe a 1oz or 2 if it is a brand new carrier.
If using a gear oil without the friction modifier add 3 oz per quart of gear oil, you can get it at any GM dealer, or Ford dealer (sorry for cursing), its the exact same suff. IF your differential is of high mileage or severe duty kick it up a notch to 85w-140 in the warm/hot weather
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