It depends on the load on the engine and the outside temperature. Heat is a transmission's worst enemy, so if you're towing trailers or moving a heavy vehicle like a motorhome or a 1-ton truck around, definitely use an auxiliary cooler. In very cold weather, the radiator-mounted "cooler" becomes a heater, warming up the fluid to at least coolant temperature and keeping condensation evaporated out. Some people add some T fittings and ball valves to isolate the cooler during below-freezing weather.
Jerry
