The short answer

The best oil is the one designed for the engine, not refined out of crude.

AMSOIL synthetics are built as lubricants. Conventional oil is what remains after crude is refined. Sulfur, nitrogen, oxygen, and metals such as nickel and vanadium come with the crude. They cannot be taken all the way out, and they do not help an engine.

What “best” has to do here

In Alaska the best oil is the one that pumps at the temperature in the driveway and still protects at operating temperature, in the viscosity the manufacturer lists. A heavier grade is not tougher. A 0W that the manual allows will be on the bearings while a conventional 5W-30 is still a gel. Signature Series 0W-20 showed 75% more engine protection against horsepower loss and wear than a leading industry standard requires, in independent ASTM D6891 testing.

Best is also an interval

The flagship is Signature Series: up to 25,000 miles or one year in normal service. XL stops at 12,000 miles or one year. OE runs the owner’s-manual interval and is still a synthetic. The best of those three is the one you will actually leave in for the miles it was built for. Mixing it with another oil throws the long interval away.

The molecular comparison, the cold-start case, and the drain limits are written out in why synthetic beats conventional.