A Transmission Cooler should be the Top Priority item on your list of upgrades.
In a manual transmission, in order to get the vehicle moving you select a gear and slip the clutch. In an automatic, you select Drive and press the accelerator. The slip action happens inside the transmission (the torque converter actually) and this is where the friction heat is generated. Frequent acceleration, deceleration, towing a van or camper increases the slip and consequently generates more heat.
Heat is the number one enemy of an automatic transmission. It destroys the additive package in the transmission fluid very quickly once the critical point has been reached. What’s more, prolonged high temperatures will harden the rubber internal seals which can lead to clutch slippage. Excess heat from towing, hill ascend and descend, beach driving etc will shorten the life of your transmission dramatically if not controlled.
Fitting a transmission cooler of the correct size and type for your vehicle will prevent this cause of early transmission failure.
In many cases, it can double or triple the potential life of the automatic transmission.