[BigAls87Z28]

There are huge differences between synthetic oils.
A true Group IV base stock is the only real synthetic oil.
The "sythetic" oils you get from Valvoline, Penz, Castrol, etc etc are highly moddified Group III stock, which is still dino oil.

You have 4 grades of oil. Most oil you buy from the big oil companies are Group III. Each company puts in different additive packages depending on its need (high milage, blends, high/low zinc)

Amsoil its a true group IV. RP used to be, but I believe due to changing up its formula to sell in retail markets (read: lower costs) that they went with a Group III.
The German Castrol Syntec is a true Group IV synthetic, but its only sold here in 0w30.

Mobil 1 and RP used to or still sell a Group IV synthetic oil but like it was said, due to the fact that it wanted retail sales, it reformulated its stuff.

As for what you should use, Mobil 1 has never let me down and I have always heard good things of Amsoil. For a while I was using Valvoline for the old Caprice, but for the Malibu Ive switched to the German Castrol.