It doesnt matter what fluid you run in the transmission, it will accept either and not care. (It's different if the transmission is really old and has lots of miles though, a full syn fluid might be the death of it.)
They just went with the times when dexron 6 came out and that was the new standard for everything GM.
Your transmission dipstick will say what was ran from the factory at it's build date. Dexron 3 is half the cost but because you'll likely never have to replace the fluid again, just run the full syn dexron 6.
