I find this (along with many others) funny.
99% of the oils sold in NA are not 100% synthetic. Mobil 1 when it came out was, later they sued Castrol for having a Group III oil being labeled as Fully Synthetic, Mobil 1 lost this lawsuit, so any oil that is Group III can claim to be Snythetic, a Semi-Snythetic oil comes from the SAME Group of oils. Only Group IV or Group V are true synethetics. I know of so few oils that meet those standards these days, that I can name the ones I know (Group IV PAO=Castrol Edge (or euro mix), Mobil 1 EP (or euro Mix), Amsoil, Royal Purple, any Euro oil claiming 100% syn (they have true standards while the US does not)...Group V are ester based, Red Line, Motuv are the only ones I know that meet that standard). Some also believe 0W-30 oils are PAO based, hence I always say they used better base stocks. Oil forumation changes often, and the makers keep there secrets. I know PP and Ultra are Group III, but I'm happy to run those products due to great additive package.
But if your not running any of those oils, you are running hydrocracked blended oil (more than likely). Semi-Synthetic shares the same base oil stocks as YOUR oil, so knock it all you want, but in the end they started as the exact same thing (maybe even the same batch), just different packages, and semi's are only 30% hydrocracked, while "fully sythetics" are just more processed mineral oil. All oil is blended, otherwise we would have SAE30, not 5W-30....
http://en.wikipedia.org/wiki/Synthetic_oil
http://motorcycleinfo.calsci.com/Oils1.html