SOME OLD INFO ON OILS
Some years back --maybe mid 90's== Consumers Reports conducted a year long (or longer) test of motor oils in NYC using regular NYC cabs. It was a very thorough test where the engines were taken apart after so many miles and all the pertinent parts were measured by the fellows running the tests who were auto engineers not amateurs.
The results were very interesting: no real difference detected between any of the American made oils that they used.
Also, they tested the belief that 3k is the magic number. No discernable change or variation found between 3K and 7 K oil changes.
The test was a 100K miles span in American made engines used normally (strange term in this city) on the streets of New York City summer and winter. Summer is hell here for any engine, particularly, if you have to hassle for a fare. By the way, taxis cannot be older than 3 years old by law.
Also, synthetic oils did not produce any tangible difference either.