Actually, the base stock is what one goes by to determine a synthetic legal label, whether here, or in Europe/R.O.W.
The API, for U.S. sold oils allows anything with a majority Group 3 and up (Group 3, 3+, GTL{Gas To Liquid, which is what Pennzoil now uses for it's Platinum Synthetic base stocks currently}, Group 4 PAO, and various Group 5s POE, etc.) base stocks to be sold as a synthetic labeled oil.
(Yes, pure straight Group 3 is still 'super hydrocracked', refined petroleum base stock.)
Whereas in Europe and elsewhere, the ACEA allows ONLY the Group 4/5 (PAO/POE) majority base stock oils can be sold as labeled 100% synthetics by law.
Most of the 'boutique', high priced premium, but not API SN/GF5 certified (due to their very high concentration additive packs, and/or the smaller companies not wanting to spend the BIG buck$ to get these certifications), are either mostly a Group 4 PAO basestock mix (Amsoil, JGD, various Motul not 300V oils), or a Group 5 POE base stock mix (Red Line, Motul 300V and a few other oils in their line, Millers NT Racing, RLI, MPT 30K, Neo, LAT, etc.).