Not disputing you that 62R is a great oil, used it myself for several years in everything including stock class motors but there is no reason to exclude 4T when the motor uses methanol. I am convinced it has an exceptional ability to retain its lubricating properties even with severe methanol contamination.
To use amsoil u either need to become a dealer or hope there's one near your area where u can get the oil and buy enough so u don't run out at the track. FHS can be found at your track if there is some body selling kart parts. There both good oils just pick whatever is more convienit for you to get the product
Idk but I ran Thor until I tore down a motor and saw the affects it left on all the steal parts so a friend talked me into the FHS oil and I have loved the results the oil looks the same going in as it dose going out. Altho I will be getting some of dovers oil and trying it soon