The infamous 0w-30 Castrol Syntec, aka GC (German Castrol) is a true synthetic. It's a PAO/Ester blend and known to be an outstanding oil, especially considering its price point (it's again on sale at $3.99)
GC is a unique breed, being imported from Germany and sold through the AutoZone chain exclusively.
5w-40 Syntec is not well known, used to be imported from Belgium (now made in US).
It differs from GC and not as highly regarded.
The rest of the Syntec line (5w-30, 10w-30 and so on) are made in the States being Group III oils.
There is nothing wrong with them but they are not considered to be a good value since those grades are priced exactly the same as excellent GC.
For more info check out the GC forum.
BTW I consider GC to be a better blend than Mobil 1.