Are "Blends" = Grp 1/2 + 3 or something else?

Not that I care about blends too much and that's probably why I've never posted this before, but... Way back in the days when Syth-blends were coming out (1993?) I called one of the oil companies to find out what the difference was between these and synthetics. I was told that they were NOT simply 'a mix' of dino and synth, but a completely new oil formula altogether. ie. you couldn't make your own 'semi-synth'. True or was I being fed a line??
Mobil advertised the original Drive Clean Blend as being a mixture of conventional base oils and PAO. For those companies where "synthetic"=Group III, you can bet their syn blends are nothing more than Group II/Group III blends (possibly with a little Group V thrown in for additive mixing). This is why Motorcraft 5w20 is marketed as a synthetic blend, while the Conoco 5w20 is not. Both are the exact same oils in different bottles and are roughly a 50/50 blend of Group III and Group II. But Conoco doesn't market theirs as a syn blend because they don't hold to the definition of Group III as synthetic.