I run GM synthetic in mine because it was free. LOL We always ran Valvoline synthetic 75-90 in the drag car. I don't remember the exact amount. I filled mine till it was just below the fill hole on the side of the third member, but if you have an aftermarket housing and third member you will have to know the exact amount or fill with the axles out until it just starts to run out the axle tubes with the rearend level.