Amsoil
I use Amsoil full synthetic. My theory is that if you set the transmission up to shift firmly, such as we do with bigger boost valves, shift improvers, and such, then the extra slip of the new fluid matters less but it's temperature resistance and long life obviously have advantages. In a stock style transmission where the manufacturer wants to please consumers with smooth shifts during testdrives while also semi-balancing long life (the length of the warranty it seems), the shift firmness is set up for one type of fluid to go with the programming or mechanical calibration the manufactuer desires. If the fluid is changed, then the whole calibration is changed as well and then the transmission will go south due to lack of total balance. Probably the reason that some builders cursed dex XI on some tranny's when they first switched as those tranny's should have had the previous fluids viscosity characteristics. Personally, my transmission shifts so hard that I couldn't imagine not using the slickest stuff out there.