I run amsoil xl 5w20 in my daily beater and have since I bought it new 10 years ago. When I get my 440 back together I intend to run amsoil z-rod 10w30. On paper, it looks good in terms of the additive package for a flat tappet cam engine. I only do 1-2 oil changes per year on my daily driver with the extended life oil. I burn no oil between changes but it's a modern engine only 10 years old so I don't think that's any sort of a relevant comparison to an old flat tappet engine.