Engine oil is used to lubricate and cool the moving parts of the engine. It protects from corrosion and is required to be changed to be maintained regularly. See the owner's manual to check the engine oil to be used. The oil lubricates moving parts to wear. Clearances between moving parts, for example, bearings and rotating shafts, are filled with oil. The parts move on the layers of oil Which reduce power loss in the engine. As the oil moves through the engine, the oil picks up heat. The hot oil drops back down into the cooler oil pan where the oil gives up heat. Therefore the oil serves as a cooling agent. Some engines have oil nozzles that spray oil into the underside of the pistons. This removes some heat from the piston heads so the pistons run cooler.