I will interject since I just did this (with mixed results) two weekends ago.
Go buy a gallon of Penzoil Dextron III from Walmart. I bought two, which was helpful.
If you just drain the pan and change the filter only, 4 quarts is all you need. If you decide to drain the torque converter as well, you'll need an additional 2.2 quarts or so.
I really haven't figured out the best way to check the dipstick, but I figured that since what drained filled a gallon milk jug, that's all I should put in.
A couple of weekends ago, I wasn't thinking that way, and based on the dipstick, figured I needed to add a couple more quarts...BIG MISTAKE! Blew the pan gasket big time, and out trickled a gallon of fluid on the driveway! So I had to do it all over again...with four quarts ONLY this time! Glad I had that extra gallon of tranny fluid.
Project for this weekend? Degrease and power-wash the garage and driveway!
