Instead of taking up most of this page getting technical about the definition of synthetic oil just take a look at what Wikipedia says: http://en.wikipedia.org/wiki/Synthetic_oil From this definition it would appear you need to be a chemist to break down most of what they are saying so I will attempt to give you the definition in the simplest terms possible.
Chances are if you have been sold a synthetic oil from a quick lube or even your dealership there is a high probability you aren’t really using synthetic oil at all. Is this the fault of the quick lube or dealership? No, not really because there is very little oversight or regulation by the API on what goes into a bottle or drum of oil that is labeled synthetic oil.
At one time, synthetic oil was made exclusively from polyalohaolefin and ester base oils. Then in 1999 that all changed when Mobil challenged Castrol when Castrol introduced an oil made from Group III base oil and called it synthetic. The dispute played out before the National Advertising Division of the Better Business Bureau and Castrol in some strange twist of fate prevailed on this issue and since then the market has really never known what a true synthetic oil is.
Now here is where the problem is and why you might be going for a very expensive ride on your next oil change. An oil with less than 50% synthetic base oil can be labeled synthetic. In fact, there is absolutely nothing to prevent motor oil marketers from labeling oils made from conventional Group I or Group II base oils as synthetic. There is currently no testing to verify the base oil content. And even if it was proven that the product contained no synthetic oil, there are no legal ramifications since no official definitions exist for the category.
The bottom line, of course, is that in many cases consumers are paying too much for oil that doesn’t measure up the high standards of synthetic as Amsoil defines the term. Chances are consumers purchasing synthetic oil for $9.99 a quart are, in all likelihood, not getting synthetic oil at all.
Not all synthetics are created equal, and it may be that not all synthetics are actually synthetic at all. Amsoil created the synthetic motor oil market clear back in 1972, and we continue the standard for performance. Our definition of synthetic doesn’t waiver. Consumers can be assured that what they see on the label is what they’ll see in the bottle. With Amsoil, people get what they pay for.
Many thanks to Amsoil Magazine for portions of this post.