First, nearly everything on the market is a blend. The only full synthetic is Mobil One and possibly Amsoil. Several years ago when initially changing oil in my Mustang (@ 8 Miles!) I searched the internet. All brands then came up with parafins and such in their MSDS (Material Safety Data Sheets) which means they are not
I keep my cars for the long haul:
-1992 Suburban 5.7 L @381,000 miles on original engine & Mobil One since ~5,000 miles. It gets 15.5 mpg at 75 mph
on the interstates & ~5,000 miles per qt.
- 2001 Mustang Cobra: @251,000 miles on original engine (4.6L DOHC) Mobil One since 8 Miles & it gets 26.5 mpg at 75 on the interstates & ~6,000 miles per qt.
Synthetics protect better by: adhering to cylinder walls longer after shutdown, have a lower friction value (yeah this means they'll give you 1 or 2% better mileage), and pump up in the engine faster on start up.
If you're going to keep one for the long haul there is little choice in my mind: Mobil One (or possibly Amsoil), the rest are fancied-up dino oils.
Go look up the MSDSs for yourself.