What does 10w40 on engine oil mean?
3 Answers
10w40 on engine oil means: 10 represents the applicable temperature of the oil at minus 25 degrees, W stands for winter, and 40 represents the high-temperature viscosity of the oil. The higher this number, the higher the viscosity of the oil at high temperatures. Engine oil, motor oil, or engine lubricant is a substance enhanced with additives in base oil, mainly containing anti-wear additives, detergents, dispersants, and multi-grade oil viscosity index improvers. The main functions of engine oil are to reduce friction and wear of moving parts, and to remove sludge and varnish from the engine. Engine oil also neutralizes acids from fuel and lubricant oxidation (detergents), improves the sealing of piston rings, and cools the engine by carrying away heat from moving parts.
The 10W-40 printed on the engine oil bottle is essentially its viscosity ID card under different temperatures. The "W" stands for winter suitability, with 10W indicating the oil can still flow smoothly at -25°C. The number 40 represents the high-temperature viscosity grade, meaning the oil maintains sufficient thickness to coat engine parts even at 100°C. For northern winters, choosing 0W or 5W oils makes cold starts easier in snowy conditions, while frequent high-speed driving or older vehicles should opt for higher grades like 40 or even 50 for better high-temperature protection. Last year during my road trip in Northeast China, I used the wrong viscosity - the engine sounded like it was chewing hardened butter during startup!
With over 20 years of auto repair experience, I've handled countless engines that seized due to incorrect oil viscosity. The numbers 10W-40 act like an oil's thermometer: the first digit (10W) indicates cold-weather fluidity (lower numbers mean better winter performance), while the latter (40) represents high-temperature protection (higher numbers mean stronger oil film). In southern regions with year-round 30°C temperatures, 15W-40 suffices, but turbocharged vehicles require 40+ viscosity. Remember that older and newer cars have different viscosity needs – my 2006 Santana runs perfectly on 10W-40, but my neighbor's new hybrid actually burns more fuel with this grade.
