What does 15w40 mean for car engine oil?

[LaReid]

Car engine oil 15w40 means: w stands for winter; the number before w represents the viscosity of the engine oil at low temperatures, the smaller the number, the better the low-temperature fluidity of the oil, and the better the protection for the engine during cold starts; the number after w represents the viscosity of the engine oil at high temperatures, the larger the number, the better the protective performance of the oil at high temperatures. Engine oil is a substance enhanced with additives in base oil, mainly containing anti-wear additives, detergents, dispersants, and multi-grade oil viscosity index improvers. Engine oil is used for the lubrication of internal combustion engines. The main functions of engine oil are to reduce friction and wear of moving parts, and to remove sludge and paint from the engine.