A Sumitomo oil cooler is a separate, smaller radiator to an engine's main radiator, which maintains an oil supply at a consistent, optimal temperature. Its purpose is to cool the oil passing through the coils, thus improving the engine and the transmission’s lifetime. They are situated in front of an engine's cooling system. Oil coolers are necessary for diesel engines (higher temperature range) and automatic gearboxes. http://www.siri-radiator.com/oil-cooler/sumitomo-oil-cooler/