What does 10w-40 mean for engine oil?

[Mockingjay]

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.

[GenevieveAnn]

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.

[Max]

After over a decade of car repairs, I've noticed that the 10W-40 viscosity grade is particularly common. The first two digits '10W' indicate cold-start performance – for example, 10W can withstand startup at -25°C, while 5W handles -30°C. The number '40' after the dash represents high-temperature protection value. When driving at high speeds with oil temperatures exceeding 100°C, sufficient viscosity is crucial to protect the engine. For northern regions, I recommend using 5W-grade oil for smoother cold starts, while 10W works fine in southern areas. I recall a customer who mistakenly used 0W-20 and ended up with cylinder scoring – older German cars especially require 40-grade oil. During maintenance checks with a viscometer, I measured that the same car model had oil films 0.3mm thicker with 10W-40 compared to 5W-30.

[StRachel]

Last year, I did some research when changing the oil myself. The 10w-40 is just a temperature adaptation code. The 'W' stands for winter, and the '10' indicates a minimum applicable temperature of about -20°C. In summer, the viscosity of 40 can protect the engine. This grade is a multi-grade oil, suitable for all seasons, which is particularly convenient. My Lavida has been running on this for 8 years, much more convenient than changing oil with the seasons. The maintenance manual says not to change the viscosity arbitrarily. Japanese cars are more suitable for 30, and 40 may increase fuel consumption. Regularly check the oil dipstick; if it turns black or emulsifies, it should be changed. I usually do maintenance every five thousand kilometers.

[StCayden]

The owner of the neighbor's auto repair shop taught me how to choose engine oil, saying 10w-40 is like double insurance. The low-temperature value of 10 ensures quick lubrication during cold starts, reducing engine wear, especially during morning ignition in northern regions. The high-temperature value of 40 maintains the oil's viscosity during aggressive driving, preventing direct metal friction. He tested it in his garage and found that older cars using 10w-40 consumed 0.5 liters less oil than those using 5w-30. However, turbocharged new cars are better suited for 20 or 30 grades due to better fluidity. My pickup truck hauls goods on mountain roads using this oil, and it hasn't needed major repairs in 6 years. Choosing engine oil depends on driving habits—those who frequently take long trips are better off with higher viscosity.