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Well the Royal Purple has friction modifier in it. But I'd still put friction modifier in it anyway. Better than spending the money to replace your posi!
Theres a mark on the differential cover that recommends which fluid should you use. I run 75 140 and it does the job. Gear oil is gear oil dude. Royal purple is overpriced crap. I run coastal with my 4.10s(yes coastal) and it serves me well. Gears holding up just fine with no problems or whine.