Well, that's an interesting question. Toyota won't say what exactly their new transmission fluid is made from so one could conclude that it is NOT a synthetic, otherwise one would assume they would say it's a FULL synthetic. The '04 Tundra's have the Type T-IV fluid from the factory. The T type has been around for several years and Toyota advises their T-IV is a unique oil composition makeup that causes smoother shifting in the Toyota transmissions. I think most people have the opinion that the Type T is just Dexron III with additional friction modifiers to make it more compatible with Toyota's clutches in their automatic transmissions. Amsoil's site states their fully synthetic transmission fluid is compatible with the Type T-IV. The '01 & '02 Tundra's use Dexron III - I'm not sure about the '03 models. I've run Amsoil in my '01 Tundra since week one and the tranny runs like a champ.