Don't let anyone power flush the transmission. Simple complete fluid replacement is a good maint measure though along with changing the spin on filter every couple of oil changes.
FWIW Castrol Transynd is considered the best trans fluid. Idk why, but used to be, may still be, that Allison wou,d double the OE warranty on commercial Alli transmissions if replaced with Transynd. (Note I don't usually get into what oil is best debates, but got in biased reccomendstion for this from an Allison service center....and I didn't buy anything from them.
