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.

 

yeah I dont see how your not having issues without friction modifier, you have it, you just dont know it

 

Use royal purple

friction modifier is a must

the parts store (autozone, advanced auto) will tell you weight.