How are motor oil grades classified?
2 Answers
Automotive motor oils are classified into full synthetic oil, semi-synthetic oil, and mineral oil. The grades differentiate the oil's low-temperature fluidity and high-temperature viscosity. Characteristics of different oils: 1. Mineral oil: The base oil of mineral oil is refined from petroleum, with irregular molecular sizes and shapes, resulting in relatively poor lubricating performance and shorter service life. 2. Full synthetic oil: The base oil of full synthetic oil is artificially synthesized, so the molecular shapes and sizes are nearly identical, providing better lubricating performance and longer service life. 3. Semi-synthetic oil: The service life and lubricating performance of semi-synthetic oil fall between those of full synthetic oil and mineral oil. Oil grade representation: Motor oil grades are typically represented by a number followed by a "w" and then two more digits, such as 5w30, 0w40, etc. The "w" stands for "winter," indicating that the oil can be used normally in winter. The number before the "w" represents the oil's low-temperature fluidity. For example, 0w means the oil can still flow normally at -35°C, while 5w means it can flow normally at -30°C. The number after the "w" represents the high-temperature viscosity, with higher numbers indicating higher viscosity. When the engine is operating, the temperature is relatively high. If the oil's high-temperature viscosity is insufficient, it cannot form a stable oil film during engine operation, leading to severe wear due to poor lubrication.
I've been repairing cars for over a decade and found that motor oil specifications mainly consist of two parts. The letter and number combination at the front represents the SAE viscosity grade, such as 5W-30 format. The W stands for Winter performance - the smaller the number before W, the better the low-temperature fluidity; the larger the number after, the stronger the high-temperature protection. Then there are API service classification symbols like SP or SN - the later the letter in the alphabet, the higher the oil quality and better cleaning performance. Nowadays many full synthetic oils display both standards. When choosing oil, you need to check both the viscosity recommended in your vehicle manual and select an appropriate grade based on the car's age - forcing high-grade oil into older engines can actually cause damage.
