Update 10-01-05: While that was true at the time I wrote the article, it is not now. All of Wal-Mart's 'S/T/ oil is now supplied by 'Superior Lubricants' out of Louisiana. 'Superior' is an industrial refiner, and while they are a good place, they are not supplying the same quality base stocks as when QS/Pennzoil was. Consequently, the 'Super'Tech' full synthetic oil is now a 'Group III' base stock. While very good, it is not quite as sophisticated as a good Group IV or Group V PAO base stock.
Synthetic Oils
Only oils that are made using the 'PAO' method are truly considered to be synthetic. Castrol Synthetic Syntec is NOT synthetic oil. It is simply a 'Group II base stock' that has been further refined using propane de-waxing and severe hydro-treating to give a better base stock. But it is still refined from paraffinic crude, as all conventional 'dino' motor oil is. Although the US FTC allows Castrol to call it synthetic, it cannot be labeled as synthetic in Europe. It has a different name over there. Mobil 1, Sheaffers, and Royal Purple are all good synthetics.
Synthetic lubricates MUCH better than any conventional oil, which of course reduces internal friction. An engine that is properly broken in and in good operating condition, and with fresh fluids in it will suffer practically NO wear and tear for many thousands of miles. Synthetic also does not have any of the damaging trace elements in it (phosphorous, sulfur, etc...) that helps to create sludge and corrosive acids when combined with the by-products of combustion.
The phosphorous, when combined with the remnants of the unburned hydro-carbons (gas), and then oxidized creates an acid that is chemically identical to battery acid. This most often leads to premature bearing failure and excessive cam wear. Since there is more actual oil in a full synthetic formula, there is more lubrication available to you. Plus, the relatively pure base stocks do not introduce any unwanted elements into your engine. 'P.A.O.' Synthesized oil is made of uniformly medium-sized molecules that all react in exactly a known fashion.
The very small, highly volatile molecules that are present in 'dino' are what escape past the rings causing high oil consumption. This is known as volotizing.
A Word About Synthetics
Only oils that are created using Poly-Alpha-Olephin based technology are considered to be synthetic. At least by anyone who knows better. The Castrol Syntec synthetic oil is actually just a much more thoroughly refined 'dino', coming from the same crude that all conventional 'dinos' come from. Even though their base stocks closely approach the performance of PAO synthetics, they still contain trace elements, and are still susceptible to the effects of temperature extremes that full synthetics are almost immune to. By the way, Castrol does not make their own oil. They never have. They don't even own a single refinery. Never have.
They simply but their base stocks from one of the commercial suppliers (Coastal-Unilube is one), buy a custom-blended additive package from an additive supplier, and have a bottler package and label it all for them. That doesn't mean that it isn't any good. It is very good. But in repeated testing, the Castrol products exhibit a wide variance in performance, the results of getting their products from many different suppliers. Syntec may be good, even excellent oil...but it sure ain't synthetic!
Kit Sullivan is a15+ year automotive lubrication engineer and a classic and muscle car enthusiast and collector hetic!