A. The API (American Petroleum Institute) categories are established through a joint effort of engine manufacturers, oil suppliers and chemical additive suppliers with oversight by API.
The purpose of the API performance category program is to ensure that a quality standard is established and maintained over the years.
The program is a voluntary engine oil testing protocol and certification process using industry-standard physical, chemical, and performance tests. If an oil can meet the minimum passing limits of these tests, it can be marketed as meeting that quality standard level, or in other words meeting the API category.
The result is an Engine Oil Licensing and Certification System list available on APIs website http://eolcs.api.org/ which lists products that qualify and meet the testing standards established by this partnership.
It also is important to know that even though an engine oil supplier may meet the basic requirements of a given API category, there is still additional headroom performance capability that can be achieved with higher performance products that go beyond that minimum. There are multiple choices within any engine oil brand. Find out what the difference is among the various oils offered by any one supplier, since performance can vary even though basic specifications are listed.