I am having my gears changed next week but I am pulling my hair out trying to find the gear lube I need. The Auburn differential I purchased calls for non-synthetic 80w90 gear oil and GM or Ford friction modifier. Is there any reason why I cant use Amsoil or Royal Purple Synthetic 80w90 with the friction modifier already added? I am planning on running synthetic unless there is some reason why I cant. I have tried to finda placethat sells either of these two by the quart but they are impossible to find in 80w90. I would like to use Royal Purple because I dont want to mess with adding the friction modifier. Does anybody know of a good place to pick up the 80w90 Royal Purple at?

 

i know you can get 70w 140 at my local NAPA