Despite their name, most synthetic derived motor oils (ie Mobil 1, Castrol Formula RS etc) are actually derived from mineral oils - they are mostly Polyalphaolifins and these come from the purest part of the mineral oil refraction process, the gas. PAO oils will mix with normal mineral oils which means you can add synthetic to mineral, or mineral to synthetic without your engine seizing up.

These bases are pretty stable, and by stable I mean 'less likely to react adversely with other compounds' because they tend not to contain reactive carbon atoms. Reactive carbon has a tendency to combine with oxygen creating an acid. (As you can imagine, in an oil this would be A Bad Thing.) They also have high viscosity indices and high temperature oxidative stability. Typically a small amount of diester synthetic (a compound containing two ester groups) is added to counteract seal swell too. These diesters act as a detergent and will attack carbon residuals. So think of synthetic oils as custom-built oils. They're designed to do the job efficiently but without any of the excess baggage that can accompany mineral based oils.

Read more: http://www.carbibles.com/engineoil_b...#ixzz2PmMHFiz0

In the old days if you had mixed oils it could create a sludge or other issues. Today since they use a Mineral Oil only as the foundation it makes the oil much easier to use and not have any bad effect is you find yourself a quart low in the desert and unable to find the correct oil. Many pure synthetics even Brake Fluids and antifreeze can not be mixed with other products or issue will happen.

The Mineral base is to make the product user friendly in a world where people do not read of follow instructions. With many things they could go all out but they have to take care of the people who will not take care of themselves.

Generally if you have high heat, cold temps, Turbo charging and want longer change conditions Synthetics are perfect.

It is kind of like Premium fuel. Not all cars need it but some do and it is up to you to evaluate. It is not going to hurt a thing and if anything will just give you added protection.

Since two of my cars need it I just use Synthetics so I only have to keep one oil on hand. Since I do my own work the added cost is not much. I just keep some 30W for the tractor.