How are motor oil grades classified?

2 Answers
AshleyMarie
07/24/25 7:55am
In the motor oil industry, oil grades are classified based on quality grades and viscosity grades. The quality grade uses the classification standard established by API (American Petroleum Institute). The API standard is the most authoritative motor oil quality classification standard in the industry. The viscosity grade uses the classification by SAE (Society of Automotive Engineers). The classification of motor oil grades is as follows: 1. API Quality Classification: 1. The "S" series represents oils for gasoline engines, with specifications including: SA, SB, SC, SD, SE, SF, SG, SH, SJ, SL, SM, SN. 2. The "C" series represents oils for diesel engines, with specifications including: CA, CB, CC, CD, CE, CF, CF-2, CF-4, CG-4, CH-4, CI-4. 3. When both "S" and "C" letters are present, it indicates that the oil is suitable for both gasoline and diesel engines. From "SA" to "SN," each subsequent letter indicates better performance than the previous one. The further the letter is in the alphabet, the higher the quality grade, making the motor oil more effective. In international brands, most motor oil grades are SF or above. 2. SAE Viscosity Classification: 10W-40 is its SAE standard viscosity value. This viscosity value first indicates that the oil is a multi-grade oil. "W" stands for WINTER, and the number before "W" represents the flow performance at low temperatures. The smaller the number, the better the starting performance at low temperatures. The number after "W" represents the oil's stability at high temperatures (i.e., the likelihood of thinning). The larger the number, the better the oil's stability at high temperatures.
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DeLaura
08/11/25 3:37am
I've been driving for over 20 years and have plenty of experience with motor oil classification. API ratings like SN and SP are alphanumeric codes representing oil quality – SP being the latest standard that offers better engine protection and emission control. SAE viscosity grades such as 5W-30 work like this: the number before W (like 5) indicates cold-start performance (lower numbers mean easier winter starts), while 30 represents high-temperature viscosity (higher numbers mean thicker oil films for better hot-weather protection). I remember using 10W-40 in my youth – the oil was too thick in summer, causing poor engine response and high fuel consumption. Switching to 5W-30 made a noticeable difference in smoothness. Another key point is regular oil changes – even premium-grade oil becomes ineffective when aged. I change mine every 5,000 km. Nowadays, new energy vehicles require special low-viscosity formulations – don't try to save pennies here at the cost of major repairs. Bottom line: always check your owner's manual and follow manufacturer recommendations for worry-free maintenance.
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