Mark is 100% correct (the only time he isn't is when he's 110% correct on these transmissions) the only time more fluid can cool better is if you have an enlarged capacity in the cooler itself and then only up to the amount of increase in capacity. Yes more fluid heats slower, it also cools slower. I bought a Pete 359 with an Allison in it that had increased sump capacity and it actually ran a higher (about 6 degree F) peak temp than after I rebuilt it an went back to standard capacity