why exactly?
I like the 4 liter and the simple tranny of 3rd gen
Under normal driving conditions, it never needs to be replaced. A few years ago Toyota actually sent out a technical service bulletin telling people to stop fucking replacing it.
In a case where it does need to be replaced, the transmission needs to be drained, refilled, then put into the specific temperature range of the vehicle you're changing it on and you have to pull the drain plug. If trans fluid streams out, it's over filled. If nothing comes out it's underfilled. If the temp range is correct and it's a steady drop from the drain plug, the level is correct. Overfilling by even a little often causes shifting issues. And underfilling can grenade the unit. The reason for the temperature range is that the fluid expands quite a bit when hot and there's no dipstick. Your Tundra has the same procedure.
If you really feel the need to change the trans fluid, it's not worth saving the hour and a half labor the dealer is going to charge you every 7 years or so. Talking to someone at Aisin (the company that builds/rebuilds the Toyota units) they say 95% of the "failed" transmissions they get in for rebuild were just overfilled.
I would suggest people ignore the once a year oil change intervals and change it twice. There hasn't been long-term reliability issues since the change, but an extra oil change a year is cheap insurance and won't exactly break the bank.
