I hear ya, but I wouldn't buy Amsoil if it was the last oil on the planet. Personal reasons. Anyway, the RS does NOT "need" 5w50 oils unless you're tracking it and running WOT for long periods. Same as the other Fords with 5w50 recommended. A 5w30 will provide more than enough viscosity for any engine other than at sustained high rpm high load track use. The more "viscosity modifiers" in any oil is bad news unless the oil is replaced at very short intervals. They thicken up with temperature and as they wear out, they clump together and result in a thick sludge instead of a flowing lubricant. The (any) engine should run the lightest viscosity possible and still provide the film strength needed. It will last longer along with the engine.I see that you are located in the states. If you are looking for an API SP engine oil in the required weight that also meets Ford’s wss-m2c-931-c, why not just go with Amsoil 5w-50? I would check with Amsoil, but on page 4 (applications section) of the following link, they seem to imply they have a product which meets both those specs:
There’s also this:
AMSOIL synthetic motor oils meet or exceed the latest performance specifications, including GM dexos1 Gen 2, ILSAC GF-6 and API SP.www.google.com.au
There's always much debate on oils due to marketing hype and "that" guy, so I do WTF I want and monitor my results over time. An occasional UOA will tell the tale quite accurately.