technically speaking: modern API SN graded conventional oil is a blend, or can be referred to as "GrpII+.
(*in other words: you can perceive it loosely as sort of a semi-syn*)
Exactly what ratio do they blend Gp 2, Gp3 and Gp4 and beyond to the mix to obtain the desired result, nobody seems to know.
One thing for certain though: after reading so many years of API /ILSAC requirements, etc. it seems certain that there should no longer be Gp1 type base oil in the mix, for it does not satisfy the volatility requirements imposed on ILSAC-GF5, or even most engine manufacturer's valvetrain tests...
Q.