Conventional mineral motor oils are refined from crude oil. By comparison synthetic oils are made from man-made organic esters and other synthesized hydrocarbons. Traditional motor oils contain impurities like nitrogen, resin, asphalt, wax, carbon, sulfur and aromatic residues. Though refining removes most impurities, some remain and may cause problems under high pressures and temperatures.
Synthetic oils are developed in the laboratory to provide the exact characteristics desired. These "designer" oils include no impurities, though like traditional motor oil will pick up some impurities during the combustion process. Synthetic oils can tolerate much higher operating temperatures, typically caused by situations where the engine must work harder, like towing, hill climbing, or heavy air-conditioning use.
While mineral-based motor oils breakdown at temperatures of 120 - 150 degrees Celsius, synthetic oils can tolerate temperatures of 230 degrees C with some up to 370 degrees C. Because synthetic oils have no impurities, they can resist oxidation up to ten times longer.
Oil's viscosity refers to its resistance to flow or "thickness." An oil's viscosity index (VI) describes how the oil's viscosity changes with temperature. While water changes from solid (ice) to liquid to gas in only 100 degrees C, oil must work consistently in a wider range of temperatures, from mid-winter below-freezing cold right up to hot engine operating temperatures. Synthetic oils have a much higher VI and thus viscosity stability.
Synthetic oils also much more freely right down to -40 degrees C since they contain no waxes that congeal at such low temperatures. At very low temperatures, mineral oils can become so thick they will not flow at all, which makes winter starts more difficult
Petroleum motor oils can lose as much as 25% of their weight when the light hydrocarbons are vaporized at high operating temperatures. Synthetic motor oils are much less volatile and are less likely to change from liquid to vapor (they may lose only 5% to 10% of their weight to vaporization) which means lower oil consumption and reduced exhaust emissions.
Though synthetic oils are more slippery and engine friction is reduced, the fuel economy from this attribute is only improved by a small percentage.
The biggest payoff really comes from reduced maintenance and repair work. While synthetic oils may last as long as 25,000 miles between changes, they pick up dirt and contaminants and should probably be changed about twice the interval for traditional motor oils. Since synthetic oils cost about twice as much as mineral oil, this keeps the oil cost the same, with lower maintenance costs plus a long run reduction in repairs. Filters should still be changed at the normal recommended interval to remove contaminants. Since we have doubled the oil change interval, replace the filter at the middle and end of this period.
Synthetic and mineral oils are available in the same grades, like 5W-30, 10W-30, 15W-50 and so forth, so they can be directly substituted. Synthetics are also completely compatible with conventional oils, you shouldn't mix them up. Make the change-over when you do an oil change.
Synthetic oils will not bring any monetary or performance benefits to a worn out engine, a high mileage engine, or one that leaks oil.