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In the US, a "synthetic" oil may be made of group III, IV, or V base stocks, while "conventional" oils are made of group I and II base stocks (mostly group II, often with some group III, IV, or V (sometimes labeled "synthetic blend" in this case), for current oils with API SL or ILSAC GF-3 ratings).
The controversy is that group III base stocks are a highly refined mineral oil base stock whose performance characteristics are similar to group IV and V base stocks, which are made "in the lab" (i.e. not from mineral oils). Group III base stocks are much less expensive to manufacture than group IV and V base stocks, but that lower cost is not generally reflected in retail pricing in the US.
Most major brand "synthetic" motor oils sold in the US are made with group III base stock. This includes most or all varieties of Castrol Syntec sold in the US. Of the major brand "synthetic" motor oils sold in the US, Mobil 1 and Mobil Delvac 1 are made of group IV and V base stocks (note: Caterpillar Synthetic Diesel Engine Oil is relabeled Mobil Delvac 1).
Castrol Syntec is probably the most well known of the group III "synthetic" oils because of a legal battle between Castrol and Mobil over whether a group III oil can be labeled as "synthetic" in the US. Castrol won that battle, but that caused some people to especially dislike Castrol for "diluting" the definition of "synthetic" (although most other major brands have also gone to group III base stocks for their "synthetic" oils).
I put a group III "synthetic" oil (Shell Rotella T Synthetic, $13 per gallon at Wal Mart) in an old Volvo with about 167000 miles on it without any new leaks. However, the rear main seal leak (which was leaking "conventional" oil) was fixed not too long before.
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