What is the difference between 15w40 and 10w40?
4 Answers
The difference between 15w40 and 10w40 lies in their low-temperature performance: 15w40 is suitable for an ambient temperature of -20 degrees Celsius, while 10w40 is suitable for -25 degrees Celsius. In 15w40 and 10w40, the 'w' stands for winter, the number before 'w' indicates the oil's low-temperature fluidity—the smaller the number, the better the fluidity at low temperatures. The number after 'w' represents the oil's high-temperature resistance—the larger the number, the better the protection at high temperatures. The function of engine oil is to lubricate and reduce wear, assist in cooling, seal against leaks, prevent rust and corrosion, and provide shock absorption.
I'm obsessed with car maintenance and have studied motor oil details extensively. The core difference between 15w40 and 10w40 lies in the first number: 15w has higher low-temperature viscosity, being slightly thicker than 10w during cold starts, with poorer flowability, potentially creating more engine resistance; whereas 10w remains thinner at sub-zero temperatures, enabling smoother cold starts. The latter "40" indicates identical high-temperature performance, offering equal engine protection under heat. Therefore, if you live in northern regions or extreme cold areas, 10w40 is preferable to minimize cold-start wear; in warmer climates, 15w40 performs adequately. Always consult your vehicle's manual—older models or diesel engines may better tolerate higher viscosity. Don't overlook ambient temperature variations, as incorrect selection can affect engine longevity, though the difference is minor compared to maintaining regular oil change intervals.
I drive year-round in the southern region and am accustomed to tropical climates, so I use 15w40 more often. It's slightly thicker than 10w40 in hot weather, providing better protection for the engine during high-temperature operation and preventing overheating wear. 10w40 has better low-temperature fluidity, making it more suitable for colder winter areas. Both have the same high-temperature rating of 40, offering equal protection—the main difference lies in the number before the 'W': 15 vs. 10, meaning slightly worse low-temperature performance. Where you live affects the choice; in my area with little snow, 15w40 works just fine. Remember to adjust according to the season—once the engine warms up, they perform similarly, so don’t overthink it.
I'm quite concerned about fuel cost. 10w40 has better low-temperature fluidity, resulting in less engine resistance during cold starts, which may save a bit of gasoline; 15w40 is slightly thicker and consumes marginally more fuel. The difference is minimal, just choose the right viscosity. The key is to use the oil recommended in the manual - don't switch randomly to avoid engine damage and increased expenses. The high-temperature performance is similar with little impact. Regular maintenance is the real way to save money.
