I've used both Amsoil and Mobil-1 in various cars. Both oils have resulted in much less oil burning and the oils do not get thick when they near the 3000 mile mark like cy former oil does (Havoline). I can't really say I see a difference between these oils in real time use. I like them both but the M-1 is less expensive.

There is a web site report where they took both brands of oil and ran them for extended mileages. As I recall they ran the oil for around 15,000 miles. Filters were regularly changed and oil replaced if it was used up. They did periodic samples of the oil and sent these to a lab for analysis. The analysis measured the metal wear products, corrosive combustion products, oil viscosity, and other oil characteristics. It was a pretty scientific study.

Both oils maintained protection for the engine over then span of the test very well. The only negative that I recall was that Amsoil tended to have a viscosity increase toward the end of the test. The M-1 viscosity was pretty steady.

From this test I can see that extended oil changes, in an engine in good repair, are not harmful to the engine. I still change my synthetic oil at 6000 mile intervals but I probably could go longer. It's comforting to know that if I have to delay changing the oil, the engine will not be hurt. That is why I put M-1 in my daughter's car. It's hard to get her to bring the car into town for servicing.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog