What does 0w40 mean?

[LaDallas]

0w40 represents the engine oil grade, where 'w' indicates that the oil can be used in winter. 0w means the oil can still flow normally at -35 degrees Celsius, and 40 is the high-temperature viscosity of the oil. The viscosity of engine oil is often identified using SAE grades, with SAE being the abbreviation for the Society of Automotive Engineers. 'W' stands for winter, and the smaller the number before it, the better the low-temperature fluidity of the oil, indicating a lower usable ambient temperature and better protection for the engine during cold starts. The number after 'W' is an indicator of the oil's high-temperature resistance, with a higher value indicating better protective performance at high temperatures.