I have always thought those two were mostly the same except for the multigrade having much better performance at low temperature. Might that mean VII's added? Anyone got good information on that?
They could be the same thing. A straight PAO base could be used to blend an SAE 20 that is also a 0w-20, so it should be quite easy to blend a 20w-20 without any VII's (so it is also an SAE20) using less expensive bases.
The last time I actually saw 20W20 or straight 20 wgt oil was when my DAD owned an ESSO Gas Station('55-'79)
He sold ESSO/Exxon Uni-Flo, Super-Flo & Plus.