When it comes to conventional motor oils, some are apparently better than other ones. People who have had motor oils tested at labs have discovered that apparently, to give two examples, Castrol and Chevron conventional oils are pretty good. How great is the difference between the best performing conventional oils and the worst performing? Is the difference so minor that it does not really matter, or is the difference significant? Could the use of a quality conventional oil as compared to a poorer quality one lead to thousands of extra miles? Are there motor oils so poor in quality that a person could safety drive only 2000 miles or so using them, while better quality oils might give 4000-5000 miles of service?