Unless you intend to sell Amsoil, the Preferred Customer option is the way to go. You get the order everything you want factory direct at dealer cost. No minimum orders or anything. You pay what I pay.
There is a membership fee of $20 per year, which I wave for forum members with their first order. This is a small cost per year. If you order 1 case of oil in the year, you will break even between the retail and PC cost. Everything after that is money saved. Put it towards truck bombs!
As for what else is available, that is kinda a loaded question. There are several 15W40 oils out there that are labeled Synthetic, but are far from it. There is no legal definition for the term synthetic, so oil makes can put the term on their bottles, even if its no different that the "other" stuff they sell. Best bet, look at the MSDS (Material Safety Data Sheet). It will tell all, because by law it has to. Shell, for instance. Their "synthetic" oil in the MSDS states that its composition of materials is 93% hydrocracked petroleum. Not synthetic, but conventional oil with a cool new label.