The owners manual states the grade, there's one for normal use and one for if you tow. Also note that with after market diffs you might need more oil vs stock...I put on ARB rear diff cover and needed almost 3 quarts. The ARB cover has a dip stick and I lined up to stock cover after i took it off and marked a line on the stick ahead of time so I knew I had the right amount of fluid.
I put on ARB's as well and they take more oil. If you are replacing the diff covers, make sure you clean off the crap that Jeep puts on them, replace them with Lube Locker Gaskets, and when torqing the bolts, only torque at 25 ft lbs each (not 35 like the manual says). I use Royal Purple 01301 Max Gear 75W-140 High Performance Synthetic Automotive Gear Oil.
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