Pros of using synthetic motor oil include improved lubrication, better protection during temperature extremes, reduced depositing of unwanted substances in the engine, improved fuel economy in some engines and increased protection of the environment. Cons include its unsuitability in some engines, such as rotary engines and many older engines requiring leaded gasoline. Synthetic oil is also higher in price than standard motor oil.
Synthetic oil provides better lubrication because it contains fewer impurities than standard motor oil. However, it reduces lubrication in some older engines that require leaded gasoline because synthetic oil does not sufficiently hold lead in suspension. Synthetic oil is also unsuitable in many racecar engines that use roller lifters because it lubricates rollers too well, causing them to slide rather than roll as they are designed to do.
Over time, standard motor oil breaks down more than synthetic oil, especially at very hot or cold temperatures. This process reduces the oil's ability to lubricate the engine and causes sludge, ash or other undesirable contaminants to build up on engine components.
Using synthetic oil is better for the environment because it contains fewer chemicals than standard motor oil. Impurities in standard oil lead to faster degradation of catalytic converters designed to reduce harmful vehicle emissions.