Eeeerrrrrreeee....opening the coffin on this thread. Just sharing my recent experience since I read this thread prior to changing trans fluid on my recent acquisition 2000 Tundra 4.7 4x4 with 230,000. I decided I wanted to run a non-syn Dex/Merc so I went with Castrol Transmax Dex/Merc. I bought 2 gallons. It works out nice that the drain and fill is right about 4 quarts (typo in early Owners manuals says 2.1qt). It looked to me like the fluid might have been original or not changed in over 100,000+ miles, it was light red/brown.
I did 2 drain and fills with a 20 mile drive in between them, I did see a bit of grit in the drain pan, a magnet wouldn't pick it up though. When I got the truck, the shifts were a bit harsh, now it is very smooth. I think I might do it one more time for good measure, maybe drop the pan and inspect the magnets.
Eric