What does 10w-40 mean for engine oil?
2 Answers
Engine oil 10w-40 refers to: 10 represents the minimum applicable temperature of the oil, which is -25 degrees Celsius; W stands for all-season use in both summer and winter; 40 represents the high-temperature viscosity of the oil, and the higher this number, the higher the viscosity of the oil at high temperatures. Engine oil, also known as engine lubricant, serves functions such as lubrication and wear reduction, auxiliary cooling, sealing and leak prevention, rust and corrosion protection, and shock absorption. The SAE viscosity classification for winter oils includes grades: 0W, 5W, 10W, 15W, 20W, 25W. The symbol W stands for winter, and the smaller the number before W, the lower the low-temperature viscosity, the better the low-temperature fluidity, and the lower the minimum applicable temperature. The SAE viscosity classification for summer oils includes grades: 20, 30, 40, 50. The larger the number, the higher the viscosity, and the higher the maximum applicable temperature.
Last time I chatted with the 4S shop mechanic and learned a lot. 10w-40 is actually the viscosity index of engine oil. The number 10 before the W represents low-temperature fluidity—the smaller the number, the better it performs in cold weather, maintaining lubrication even at -25°C. The number 40 refers to high-temperature viscosity, meaning the oil film becomes thicker when the engine heats up, offering stronger protection. It's like how we wear fleece-lined underwear in winter and switch to thin T-shirts in summer—engine oil must also be chosen based on temperature. Older cars are particularly suited for this type, as higher viscosity can seal gaps in worn cylinder blocks and prevent oil burning. However, using it in newer cars might be slightly less fuel-efficient, which is why many modern cars now use 20-grade oil. Always check the manual's recommendation when choosing oil—don’t switch randomly.
