Viscosity is the key word no one has mensioned yet!
Meaning the oils ability to flow, affected by temp as oil is heated its viscosity lowers. Allthough it may seem like its "thickness" or "heavyness" it is not always the case.

Anyways for these engines I think its a case of following what other guys have been successfull with, I use silkeolene pro s 0w50. Just because thats what it was run on before i bought it. Dont know what everyone else uses right enough.
As for additives I saw a demo for that Active 8 stuff the other week and it really had me sold! Anyone use this? Or at least saw the guy demo it, its awsome.