I'll say it again for the 100th time.
THE API IS A LOBBY GROUP FOR THE PERTROLEUM INDUSTRY!
This is why GM came out with their own standard for the corvette.
This is why I look for european standards for my VW as opposed to the API, ACEA standards are much higher than what the minimum API requires.
Now if the API came out with a Tiered rating system, where there is a minimum standard, but then another catagory for better oils, that would make a little more sense, But come on. How in the **** can $.69/qt oil be as good as Mobil 1 which is $5/qt? IT'S NOT!
If it were me, I would not lose any sleep over using Amsoil, Mobil 1, Redline, or any other top quality oil, API or not. As long as the oil is made for the proper specifications, you are good to go.
BTW, API certified oils are normally only for automobiles. Most Industrial equipment lubes do not have API information. Guess what, Industrial equipment if far more expensive than your little ford mustang engine.