A heat exchanger for cooling oil. Most automatic transmissions are equipped with an oil cooler that's located inside the radiator. Since the radiator usually runs close to 200 degrees, the amount of "cooling" this kind of setup provides is questionable. An aftermarket oil coolers that can be installed outside the radiator

can provide much better cooling, and is recommended for towing or high performance applications. Except for air-cooled engines (older Volkswagens for example) and race cars, most engines do not use an oil cooler for the engine. The engine's cooling system is usually adequate to keep oil temperatures within safe limits.

Radiators are used for cooling internal combustion engines, chiefly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of such an engine. They operate by passing a liquid coolant through the engine block, where it is heated, then through the radiator itself where it loses this heat to the atmosphere. This coolant is usually water-based, but may also be oil. It's usual for the coolant flow to be pumped, also for a fan to blow air through the radiator.