I changed the oil in my tranny and rear end this past weekend. I used Amsoil Synthetic ATF for the tranny. Amsoil synthetic 75W90 gear lube, and "stage 1" friction modifier for the rear. It made a huge difference in the feal of my tranny. its so smooth now and shifts a lot easier. i didnt really notice much in the rear end, it worked fine before and it still works fine. The oil that came out of there though was a thick black sludge lol so im sure its helping some. I have heard of some people complain about grinding after switching to synthetic ATF, but it hasnt been the case for me , it feals like a whole different transmission.