The main purpose of the engine lubrication system is to reduce wear of rubbing engine parts: pistons, cylinders, crankshaft bearings, gas distribution mechanism. Engine oil also performs a sealing function - it seals the gap between the piston and the cylinder walls, where, despite the special rings, there is a gap of thousandths of a millimeter. In addition, the oil also performs a cooling function. When fuel is burned, only a third of the energy received is used to perform a useful action. The rest leaves the engine in the form of heat along with the exhaust gases. Another part of the heat energy is removed through the cooling system and oil. For example, on the one hand, heat is removed from the piston through the rings and the oil layer and further through the cylinder walls by the coolant. On the other hand, the piston is directly cooled by the oil sprayed in the crankcase. In addition, all bearings of the crankshaft and camshaft, etc. are cooled with oil. The heat absorbed by the oil is then discharged to the atmosphere through the oil pan.
When designing engines, other additional requirements are imposed on the oil: for example, it should not evaporate at a high temperature that occurs on the working surface of the cylinders, and the oil film should not be broken under these difficult conditions. The combustion of oil should not be accompanied by the formation of soot - after all, oil consumption is due to the combustion of part of it during engine operation. The oil should also wash away soot and grime so they don't build up in the engine. It should not lose properties over time, and should also prevent engine contamination, etc.