im going to be changing out my diff and transfer case fluids probably this weekend since they are past due. im just curious what brands of oil you guys prefer to use and what weights, not just for the differentials, but motor transmission and transfer case. i know that the t-case uses atf. when i had my jeep, i used pennzoil in everything except the tranny, i put royal purple in that. as of right now, the motor in my truck has castrol in it, not my choice, the shop i had it done put it in.(i was too busy at the time to do it myself) i hear alot of hype about amsoil being great but is it worth it? i wondered whether i wasted money buying royal purple for my jeep if regular pennzoil or valvoline fluid would have done the same. anyway, what do you guys prefer to use and did you notice a difference with it compared to other oils? also, where did you buy it?