What is the difference between 5W40 and 15W40 engine oil?

[LaMaddox]

The main difference between 5W40 and 15W40 lies in the oil's flowability at low temperatures. Firstly, the "W" stands for WINTER, and the number following it indicates the viscosity, which is the same for both 15W40 and 5W40. The number before the "W" represents the minimum temperature the oil can withstand. The smaller the number, the better the low-temperature flowability, meaning 5W40 is suitable for lower temperatures. 15W40 is suitable for regions with higher temperatures in the south, frequent aggressive driving, hilly or mountainous areas with heavy loads, and vehicles that run for extended periods, such as taxis, vans, and light trucks. However, certain brands like Japanese Toyota, Honda, Nissan, Mazda, Mitsubishi, Korean Kia and Hyundai, and American GM, Ford, Dodge, and Jeep models should not use the higher viscosity 15W40 oil but must use the manufacturer-specified 5W40 oil. Engine oil viscosity changes with temperature, so it's essential to pay attention to its performance under both low and high temperatures. For example, in 5W-30, the first number indicates low-temperature flowability, with 5W representing the ability to withstand -30°C. The smaller this number, the better the oil's flowability at low temperatures. 0W is -35°C; 5W is -30°C; 10W is -25°C; 15W is -20°C; 20W is -15°C; and 25W is -10°C. This ensures smoother cold starts for the vehicle. The second number represents the oil's kinematic viscosity at 100°C. A higher number means the oil can maintain its viscosity better at high temperatures, indicating better lubrication performance under high-temperature conditions. Generally, better low-temperature flowability ensures smoother cold starts and better engine protection. Engine oil viscosity should not be too high or too low. If the viscosity is too low, it can increase engine wear. If it's too high, it can increase engine resistance during operation, leading to reduced power and increased fuel consumption. When the engine is running, the oil forms a film on the surfaces of engine components, reducing friction and lowering resistance. If the oil viscosity is too low, it cannot form a stable film, leading to increased engine wear. For full synthetic oil, the recommended replacement interval is one year or 10,000 kilometers. For semi-synthetic oil, the interval is shorter, requiring replacement every 7,000 kilometers or six months. When choosing engine oil, consider the following: Select oil based on the engine's requirements. There's no need to use overly advanced oil for engines with lower requirements, nor should lower-grade oil be used for high-performance engines. Multi-grade oil is recommended due to its cost-effectiveness, longevity, and efficiency, providing better engine protection. Due to its characteristics, multi-grade oil may appear darker or have lower oil pressure compared to conventional oil, which is normal. Since engine oil significantly impacts performance and lifespan, always follow the vehicle's manual for selecting the correct series, performance grade, and viscosity grade. The oil recommended in the vehicle manual is based on the engine's performance and the regional climate, providing guidance with a significant safety margin.