There was a lawsuit decades ago, where Castrol was the first company to call a group III oil synthetic.
Mobil 1 at the time had PAO's and Ester's in their synthetics, and Mobil 1 sued Castrol and lost.
The judge ruled Castrol had proved that group III went through enough transformations from Dino oil to have it's molecules more aligned, and could be called synthetic. At that point, Mobil 1 was in a position where their competitor could produce the Group III oil so much cheaper than Mobil 1's PAO / Ester synthetics, that Mobil 1 had to cheapen it's synthetic's to group III to be competitive. Ever since then, nearly all synthetic oil is group III (which is a bad thing for consumers in my opinion).