API is the product mark of the " American Petroleum Institute " (commonly known as donuts).
Its primary purpose is to develop standardized working standards for the safety, reliability and interchangeability of oil and gas extraction. It is also an accredited standards development organization of the American National Standards Institute.
The standards set by API are used in many countries for oil, natural gas chemical and extraction machinery technologies. Because the process is standardized and difficult to pass, products with the API certification mark are widely considered to be of high quality.

Conclusion:

The API grade is mainly based on the operational performance of the engine oil, whether it meets the environmental requirements... etc. In all aspects of the strict standard process of selection to the final approval of the certification, the Institute will also internally conduct sampling inspection of various brands of oil from time to time. If any of the oil samples do not comply with the standards, the certification will be cancelled!

Such strict regulations will really make it more difficult to obtain the API.

Note: Corrida oil has also achieved the highest SP grade currently certified by API.