I'm using Amsoil front to back. I have the 75W90 in the front diff, 15W40 in the engine, ATF in the transmission, ATF in the transfer case, and 75W140 in the rear axle. I completed the switch over at about 60,000 miles. At that point my fuel mileage went up 1. 5 mpg. I don't think you can go wrong with synthetics.
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