Actually, Motorcraft's MSDS gives a decent idea of what is in the blends. Their 5w30, for example, is listed as "60%-100% of petroleum distillates, hydrotreated, heavy paraffinic" and "10%-30% of ...solvent dewaxed...". There are also CAS numbers that one could look up.
So it seems motocraft blend is largely group III (though I guess could be some group II/II+ blended in) along with a small percentage of group I.
Good oil. It's running in my family's corolla right now.