What is the difference between 15w40 and 10w40 engine oil?

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The difference between 15w40 and 10w40 engine oil lies in their low-temperature performance: 15w40 is suitable for environments with temperatures as low as -20 degrees Celsius, while 10w40 can be used in environments with temperatures as low as -25 degrees Celsius. In 15w40 and 10w40, the 'w' stands for winter, and the number before the 'w' indicates the oil's low-temperature fluidity—the smaller the number, the better the low-temperature fluidity. The number after the 'w' represents the oil's high-temperature resistance—the larger the number, the better the protective performance at high temperatures. The functions of engine oil include lubricating and reducing friction in the engine, assisting in cooling, sealing to prevent leaks, preventing rust and corrosion, and cushioning shocks.