Synthetic oil, which is manmade in a laboratory as opposed to being derived from crude oil, is more stable at a variety of temperatures and has a more consistent molecular structure. Because of this, synthetic oil helps engines run cooler, provides better lubrication at higher temperatures and lasts longer.
Conventional oil refined from crude oil contains impurities and hydrocarbons with different chain lengths. The shorter hydrocarbons burn off at higher temperatures, causing the oil to thicken during use and degrade more over time. A common myth about synthetic oil is that it should be avoided in older, high-mileage cars and that switching between conventional and synthetic oil eventually causes leaks. These rumors occurred when synthetic oil first came to market, and the manufacturers didn't pay attention to seal compatibility; this problem was addressed and is no longer an issue as of 2014.