OK lets talk about Amsoil myths, truths, facts, and fiction. First I am not saying Amsoil is bad its not its a good oil. But here it goes. Amsoil has a problem in the industry right now in the auto industry they cant pass the API standards.
Key Limitations For API Licensed Formulas
Phosphorous content - .10% maximum
(API SL; 0W-20, 5W-20, 0W-30, 5W-30, 10W-30 viscosity grades, only)
NOACK volatility - 15% maximum
The prevalent sources of phosphorous in motor oils are additives called zinc dithiophosphates (ZDTPs). Currently, these versatile additives act as oxidation/corrosion inhibitors and aid in the ability of a lubricant to reduce wear. The automobile manufacturers, however, have demanded that lubricants contain a maximum of only .10% phosphorous. Their reason is that some manufacturers believe that higher phosphorous content levels will poison the catalytic converters on their cars before they reach 150,000 miles, which is the number of miles that their vehicles will be required to pass EPA emission standards.
Now obviously Amsoil has put a their own positive spin on this problem on their site:
AMSOIL API Licensing
However the bottom line is take out the phosphorous if you want to pass or don't if you don't. So Mobil One passes they don't. Also personally I don't want to take the risk that at 150k I will need a new emissions system. They are expensive. That is just me...... Also Don't let Ford GM Chrysler find out you used a non api oil they will void your warranty even though I know Amsoil says they will pay the tab thats the last thing you want to ever have to deal with. So that is why they cant post it. So on a tractor with no emissions it doesn't matter.............OK
Now as far as superseding New oil always supersede old oils as far as quality and ability. The only exception to this rule is if you have a 50s-60's car and want the Zinc that was originally in the oil for lubrication of the valve train. New oils don't have it anymore. HOWEVER If you take your late model car and put Shell Rotella T 15W40 in it you will have the original formula and better that it was meant to run on. Most people don't realize that. Again don't try this in a emissions car.
Type in API on the INTERNET and you can get all the codes and the years and the superseding codes and current codes.
I hope this helps with the pain................