I've never touched Amsoil. It was impossible to find back when I first heard about it and I'm not a fan of their distribution model (MLM). I only run Mobil 1 Fully Synthetic in all my vehicles. 324,000 miles on my 2007 Suburban and 185,000 on my 2002 Dodge Ram 1500. I replaced the valve cover gaskets on the 2007 Suburban last year and there was not a single bit of buildup anywhere. Other than having the slight yellow color, it was as clean as the day it rolled off the factory floor.
My brother runs it in his plow trucks. Claims he doesn't have to add oil between changes like he did with other oils.
I run Mobil 1 because it is so readily available. Have driven a couple of GM 3800's well over 300k with the Mobil 1.
I do use the Amsoil ATF. 2 people that I know with heavy diesel pickups with transmission temperature sensors have told me that the temperatures in their transmissions dropped significantly with the Amsoil ATF when they were towing.
My 2500 HD with an Allison trans and fluid temp sensor/gauge, never goes above 180-deg using Valvoline Max Life and regular external filter, no matter what I pull. I pull a 5500k camper through the mountain passes in the spring & summer! .