Ideal ATF Temp?

The allison truck automatics that I work on use the same dexron III fluid as cars and even with large coolers the fluid runs 200 degrees and in the summer reaches up to 300 on the trucks with retarders on them. Allison says this is okay and if we change it yearly we have no problems. So 170-180 is okay but the fluid will be okay even if it gets up to 225-250 and there also is synthetic dexron which can take 330 temps.