I read a study on oil changes in the '90's or so done on New York cabs. The study varied the brand of oil and the oil change interval. Engines were taken apart and analyzed after so many miles. The conclusion was that the brand of oil did not matter. Regular changes made all the difference. I think they advocated every 3K miles.
Since then I have bought the cheapest oil of the proper grade. When full synthetic came along I started using it, with changes every 8-10K miles. No engine problems, but we don't keep cars much past 120K miles. My 2003 Exp FX4 went 150K before selling it. Ran fine on 10K oil changes with the cheapest full synthetic I could find on sale. If you use regular oil, 3-5K oil changes are best. Blends, 5-7.5K between changes. Just my opinion and experience.
On further research, not exactly as I remembered it:
Saturday, June 29, 1996 - Page updated at 12:00 AM
Auto Report
Two-Year, N.Y. Taxi Test Proves Motor Oils Alike, Magazine Says
Seattle Times News Services
NEW YORK - Premium motor oil and fancy additives are nothing more than slick sales, Consumer Reports says.
The consumer watchdog tested 20 types of motor oil under the most grueling conditions - in a fleet of 75 New York taxis - for nearly two years.
The results: There is little difference among brands as long as the container carries the starburst symbol, which means the oil meets a standard set by the American Petroleum Institute for additives needed to keep modern engines running.
The study also found:
-- Changing a car's oil every 7,500 miles, or at the automaker's suggested interval, is sufficient. Although some mechanics recommend oil changes every 3,000 miles, that only wastes money and adds to the nation's oil-disposal problems.
-- Expensive synthetic oils worked no better than conventional motor oil in the taxis, which drove 4.5 million miles in stop-and-go traffic. But synthetic oils may be worthwhile for cars in extreme driving conditions: very hot or very cold climates, or in cars with a high engine load.
-- Additives such as Slick 50, STP Engine Treatment and STP Oil Treatment showed no significant reduction in friction and engine wear compared with engines using the same oil without the additives. Consumer Reports said additives are unnecessary as long as the oil used meets the API standard.
The findings are reported in the July issue of the magazine.