Basically there are two classes of popular synthetic base stocks used in motor oil. Group III are petroleum bases, but are so highly refined that they can be legally classified as synthetic even though it's not a synthesized product. Most 'synthetics' on the shelf are really Group III petroleum bases. Group IV/V base stocks are true synthetics - in other words fully man-made base stocks. Group II stocks are less refined petroleum base stock and what you get when you buy 'conventional' oil.
Lotsa people think that synthetic on the bottle should mean just that - a man-made stock which is superior in all aspects. They think highly refined petroleum stocks labeled as synthetics are a cop-out, and that you aren't getting what you are paying for when shelling out for 'synthetic' oil. For example, Mobil 1 used to be a Group IV full-synthetic PAO base, and has since switched to Group III hydrocracked petroleum base, causing Mobil 1 fans to create quite an uproar about how Mobil 1 was copping out and how dare they do that to their customers.
In reality, there is a pretty big performance gap between Group II conventional base stocks and Group III/IV/V stocks. But the gap between petroleum Group III stocks and true synthetic Group IV/V stocks is so narrow it's hardly measurable - especially when you consider a lot of overall fine-tuning of performance qualities come from the additive package. When your base stocks perform so similarly you're talking minutiae and splitting hairs. Hence the legal classification on Group III base stocks as synthetic even if it ain't. True synthetic Group IV/V stocks are also MUCH more expensive to manufacture for almost no performance benefit, which is why so many synthetic oils have switched to Group III petroleum base.
FWIW When I say performance we're talking standard stuff like wear, viscosity shear, thermal stability, pour point and flow qualities, useful life, etc etc.
So although you'll hear the internet geniuses raise a fuss, their neighbor's Wal-Mart Group III synthetic is working and protecting just as well, and lasting as long, as their $10-quart magic pixie dust Group IV oil. Unless you're going to go 20k miles without a change, you'll never receive the benefit.
Group II 'conventionals' to synthetics worth the performance difference? To me, yes. Nitpicking over stock types in synthetics? Not so much.
They are developing even more refinement processes right now for petroleum stocks that will exceed the performance specs of the best Group IV/V oils, and they will still be much cheaper.
Don't think this is an easy pill to swallow for the vendors of super-premium synthetics like Amsoil, they'll argue this stuff until they're dead. Back when those types of oils were introduced, there was a real need for them because petroleum refinement was nowhere near where it is now. But, times are a changin'.
Last edited by BADGER; 08-24-2009 at 08:38 AM.