Engine oil is basically lubricant. Lubricants reduce friction. Friction and heat are generated in the process of moving each part of the engine interacting with each other. Engine oil forms a membrane on the surface of parts that are in contact with each other to reduce the heat generated by friction. As a result, the engine is protected. The oil itself also takes the heat away from the engine and acts as a coolant to prevent engine overheating.
The antioxidant effect that prevents rusting is also great. The oil coating on the metal surface of the engine oil prevents oxidation of the metal by preventing it from being directly exposed to oxygen. It prevents rusting. Add a corrosion inhibitor to the engine oil for a more positive effect. A detergent is also added, which greatly reduces oil sludge accumulation.