Synthetic base stocks are more expensive because they are "synthesized" in a petrochemical process rather than being distilled out of crude oil.

Also, the profit margins are higher and the industry spendings millions per year trying to convince John Q. Public that synthetic oil is "better".

In reality, synthetic is really no better than conventional mineral based motor oils assuming you follow a traditional oil change schedule. Because synthetic base stocks are more resistant to oxidation, they have longer service life and are usually accompanied with an OE oil monitoring system, but using synthetic
in an older car and changing it at the normal interval recommended for a conventional oil is a waste of money, but without a monitoring system you don't really know how long you can go.

The most critical aspect of a motor oil is the ADDITIVE PACKAGE and all API SL oils have about the same additive package regardless of base stock. Since most of these additives
are consumed during service, synthetic oil with longer change intervals is certainly NOT an improvement.

The basic advanages of synthetic are longer service life due to due slower base stock oxidation rate and a wider viscosity range with little or any VI improver, which break down with use. That's it.

Duke