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 Joe public can add synthetic to his mineral, or mineral to his synthetic without his car engine seizing up (although I've heard Mobil 1 is actually made by reformulating ethanol).
These bases are pretty stable, and by stable I mean 'less likely to react adversely with other compounds.' They tend not to contain reactive carbon atoms for this reason. 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.
