Not exactly a myth, but not exactly true either. What most people don't realize is that the term "synthetic" isn't really defined and there are a wide variety of oils on the market that can be labeled "synthetic". Each behaves differently and it really isn't accurate to make blanket statements about how a synthetic performs without knowing what its basestock is... and that is getting harder and harder to determine as oil companies become more tight lipped about their basestock.
The API has not come out and defined what is "synthetic", but rather, classified oils into five major groups.
Group I base oils are the least refined of all of the groups. They are usually a mix of different hydrocarbon chains with little or no uniformity. While some automotive oils use these stocks, they are generally used in less demanding applications.
Group II base oils are common in mineral based motor oils. They have fair to good performance in the areas of volatility, oxidation stability, wear prevention and flash/fire points. They have only fair performance in areas such as pour point and cold crank viscosity. Group II base stocks are what the majority of engine oils are made from. 3000 mile oil changes are the norm.
Group III base oils are subjected to the highest level of refining of all the mineral oil stocks. Although not chemically engineered, they offer improved performance in a wide range of areas as well as good molecular uniformity and stability. By definition they are considered a synthesized material and can be used in the production of synthetic and semi-synthetic lubricants. Group III is used in the vast majority of full synthetics or synthetic blends. They are superior to group I and II oils but still have limitations. Some formulations are designed for extended oil changes.
AMSOIL XL Motor Oils, Castrol Syntec and many others fall into this category.
Group IV are polyalphaolefins (PAO) which are a chemically engineered synthesized basestocks. PAOs offer excellent stability, molecular uniformity and performance over a wide range of lubricating properties.
AMSOIL SAE Synthetic Motor Oils and Mobil 1 primarily use group IV basestocks (though recent info posted on the oil forum indicates that Mobil 1 is also now using a significant proportion of Group III basestock). PAO is a much more expensive basestock than the highly refined petroleum oil basestock of Group III.
Group V base oils are also chemically engineered stocks that do not fall into any of the categories previously mentioned. Typical examples of group V stocks are Esters, polyglycols and silicone. Redline uses an ester basestock.
In the 90s, Mobil filed suit against Castrol for falsely advertising Syntec oil as synthetic, when in fact it contained a Group III, highly hydroprocessed mineral (Dino) oil, instead of a chemically synthesized (group IV or V) basestock. Due to the amount that the mineral oil had been chemically changed, the judge decided in Castrol's favor. As a result, any oil containing this highly hydroprocessed mineral (Dino) oil (currently called Group III basestock by the American Petroleum Institute) can be marketed as a synthetic oil. Since the original synthetic basestock (polyalphaolefin or PAO) is much more expensive than the Group III basestock, most of the oil blenders switched to the Group III basestock, which significantly increased their profit margins.
PAO basestock causes engine seals to shrink slightly (not swell as posted above). Ester basestock causes seals to swell slightly. As a result, in the early days of synthetics, seal leakage was common (due to most early synthetics using a PAO basestock) and led to the widely held view today that synthetics will cause seal leaks. However, AMSOIL has been blending synthetics for over 30 years now and today they use a basestock that is predominantly PAO based, but with an ester additive which counteracts the seal shrinkage of the PAO.
For those synthetic oils using a Group III basestock (the majority of synthetic oils on the market today), their performance with regards to seal shrinkage shouldn't notably different from that of mineral based oils as the basestock is just highly processed mineral oil.
And to confirm what Noahsdad posted above, my wife's Chevy Trailblazer with the 5.3L V-8 has been using
AMSOIL XL Synthetic 5w30 since it was new is showing wear on the average of 11-75% below Blackstone Labs universal average for that motor family (and this with a 5817 mile oil change interval:
2003 Chevy Trailblazer Used Oil Analysis AMSOIL XL 5w30
:cheers: