diesel engines start with c, and the quality grades are arranged in alphabetical order, and the higher the letters, the higher the grades. The API levels of diesel oil include CD, CF, CF-4, CG-4, CH-4, CI-4 and CJ-4. The methods of distinguishing diesel grades are as follows: 1.C stands for diesel. Starting from CA, CB, CC, CD, CE, CF, CF-4, CG-4, CH-4, CI-4 and CJ-4, the later the letters are arranged, the higher the quality grade of the oil. Generally, diesel engines can use CD-grade oil, which shows that supercharged diesel engines require higher oil quality. -4 indicates that the oil is suitable for Grade 4. 2.API engine oils are divided into two categories. The series at the beginning of' s' indicates gasoline engine oil with specifications of APISA, SB, SC, SD, SE, SF, SG, SH, SJ, SL, SM and SN. C' series at beginning indicates diesel engine oil. When the words' s' and' c' exist at the same time, it means that the oil is of the general type of gasoline and diesel; 3.SN grade is the best grade oil at present. SJ is the standard in 199s, SL is the standard in 21, SM is the standard in 24, and SN is the standard in 21.