Amsoil will work. I have been doing drain-and-fills using Mobil 1 ATF every 15k. Some will tell you to flush, but I prefer the gradual drain-and-fill method, especially with higher mileage. Yours is not that high mileage, but is pretty old fluid, so that is the way I would go. Drain from the pan. Fill back up through the dipstick with 4 quarts. Easy. Do this 3-4 times with some mileage in between and you will have replaced all of the fluid.
I heard it would not be a good idea to mix synthetic tranny fluid with the original fluid that came in our trucks. Are you sure this would be a safe thing to do to our cranky trannys? I would think a complete flush with synthetic would be the way to go... no mixing of synth and dino...
No issues mixing. Been doing that with Mobil 1 for 45k miles. Just don't expect a mixture to perform like a synthetic. Doing several drain-and-fills over a period of time brings up the detergency a little more gradually. Keeps all the crud from turning loose at once and clogging up the transmission.
On Tundras prior to 2003, you certainly can use synthetic ATF. My V-6 Camry is a 1996 model with 188,000 miles. I use Mobil 1 ATF, and have for over 100,000 miles. I bought the little car new and with the Mobil 1, it shifts s-m-o-o-t-h...never had problem one with that great A-W transmission.
Mt Tundra is a 2003 model, so I cannot use the Mobil 1---it calls for T-IV, so that's what I use in it.