On my old Cavalier, it has a 3T40 3-speed auto. I just changed the fluid. I used Castrol Dex/Mer I got on clearance. I always install 4 quarts after dropping the pan and filter as it always comes out perfect. Thing is I don't know the actual level of fluid in the transmission. (Transmission dipstick, meet positive post.) After the change out once it gets hot, it shifts like dog poop, when it does. Sometimes it will decide to stay in 1st or 2nd for awhile. Sometimes, depending on circumstances, it will slip hard coming off the line. Feels like a neutral drop. Now, I had been running Type F in it for awhile because it would slip a little in 2nd, but this is so much worse.. I added another 1/3 quart or so in case it was low initally, but its still doing the same thing.