"Quality " is not the right word, is it performance or properties. For the US ( which most of the world follows) , the API defines several tests . The API service numbers ( like SN, SM, SL ,and SJ ) Define which series of tests the oil has passed. An oil may pass more than one set of tests. When I was familiar with them there were both engine tests and bench tests. API has auditors that check documentation and the quality control programs at the manufacturers for compliance. It spoils some of the advertising departments' fun but brand "X" marked SJ is at least as good as brand" Y" marked SJ, etc. However,one oil may pass a test by a greater margin than another oil. But the only way to know that is to look at the manufacturers data, to which very few people have access. So you can look at your car manual and if it says SJ , go buy the best looking oil can marked SJ ; Or you can read the requirements for the API tests and try to pick some other type that you think is better for your car. Also , I suggest pick the viscosity .like 5W30 . that the manual lists.