Even though i have not used Amsoil products before as they are rare and ridiculously expensive here in Australia, with other excellent cheaper oils available, from a specs point of view, they are excellent. Nearly all their Petrol oils carry a CI-4 rating or higher to accomodate for both fuels. It should be like that with more oils. From its HTHS and viscosities, it seems like a pretty thin 20w50 as well compared to others i have used and seen.
Pennzoil High mileage 20w50 has some better HTHS, noack etc, but see the largee visco differences ? Yes AMSOIL carries the higher diesel rating. Better all round oils the Amsoil.
