It takes 1.8 quarts of a good quality GL-5 or GL-6 gear lube, but the exact viscosity is not as important in the axles as is it is in the engine. 70W-80, 75W-90 etc. is fine, or if you tow a trailer with your Jeep, a higher viscosity like 80W-120 to 85W-140 is recommended. Personally, I prefer and run a non-synthetic in my axles. I recently switched to 85W-140 Torco GL-6 gear lube (sold by Currie so it has Currie's label on it) in my axles.