What does the oil grade 15w-40 mean?

[DelArabella]

15w-40 oil represents the oil grade. The 15 indicates the minimum operating temperature in winter is -20°C, meaning the smaller the number before W, the lower the low-temperature viscosity, the better the low-temperature fluidity, and the lower the applicable minimum temperature. The 40 represents the high-temperature viscosity in summer, meaning the larger the number after W, the higher the applicable maximum temperature. Additional notes on adding oil: 1. Higher oil viscosity does not necessarily mean better engine protection. High-viscosity oil not only negatively impacts vehicle fuel consumption but also fails to protect the engine effectively. Oil also has a cooling function, and blindly using high-viscosity oil will inevitably weaken the engine's heat dissipation performance. 2. Oils should not be mixed. The additive components in oils from different manufacturers vary completely, with significant differences in chemical compositions and product sequences. Mixing two brands of oil can cause chemical reactions under the engine's high temperature, such as the formation of flocculent substances or increased viscosity.