I became an Amsoil dealer after using their products and liking them so much.
I don't have a trans oil cooler on my 04 CompG and use Amsoil ATF; the temps are the same as my 98 GTP that did have a GM cooler and used your regular Mr. Transmission ATF. The trans has a 3000 stall converter that those without an oil cooler have typically had heat issues with. Amsoil says to expect a drop in trans operating temperatures and thusfar I'm inclined to believe them, it was worth the money to protect a several thousand dollar investment.
Those that don't spring for Amsoil I normally refer to their local GM dealer and pick up some GM Dexron VI. In past I've also had good experiences with Pennzoil ATF.
Castrol synthetic ATF I had terrible experiences with, lots of slippage that was immediately cured with a fluid flush back to regular ATF.