What does the engine oil grade 5w40 mean?

[VanRuth]

The 5w in the engine oil grade 5w40 represents the viscosity of the oil under low-temperature conditions; 40 indicates the oil's high-temperature resistance index. The 'w' stands for winter, and the smaller the number before it, the better the oil's low-temperature fluidity, meaning it can be used in lower environmental temperatures. The engine oil grade consists of two parts: classification and viscosity specification, typically indicating viscosity and quality. The primary functions of engine oil are to reduce friction and wear on moving parts, remove sludge and varnish from the engine, neutralize acids produced by the oxidation of fuel and lubricant, improve the sealing of piston rings, and cool the engine by carrying away heat from moving parts.