Advantages The technical advantages of synthetic motor oils include:
- Measurably better low and high temperature viscosity performance
- Better chemical & shear stability
- Decreased evaporative loss
- Resistance to oxidation, thermal breakdown and oil sludge problems
- Extended drain intervals with the environmental benefit of less oil waste.
- Improved fuel economy in certain engine configurations.
- Better lubrication on cold starts
The disadvantages of synthetic motor oils include:
- Initial costs are usually two to four times greater than petroleum-based oils, though at one time, man-made oils cost ten times more than petroleum. Initial costs are often mitigated by extended change intervals, which individuals may confirm through used oil analysis (UOA).
- The lower friction may make them unsuitable for break-in (i.e. the initial run-in period of the vehicle) where friction is desirable to cause wear. Improved engine part machining has made break-in less critical than it once was though. Many modern cars now come with synthetic oil as a factory fill.
- Potential decomposition problems in certain chemical environments (industrial use dominantly)
- Potential stress cracking of plastic components like POM (polyoxymethylene) in the presence of PAOs (polyalphaolefins).
- Potential on some older pushrod race engines with roller lifters for the roller itself not to spin with camshaft movement, but rather slide while the roller itself remains either stationary or at a lower circumferential speed than that of the camshaft lobe
- In July 1996, Consumer Reports published the results of a two year motor oil test involving a fleet of 75 New York taxi cabs and found no noticeable advantage of synthetic oil over regular oil. In their article, they noted that "Big-city cabs don't see many cold start-ups or long periods of high speed driving in extreme heat. But our test results relate to the most common type of severe service - stop-and-go city driving." According to their study, synthetic oil is "worth considering for extreme driving conditions: high ambient temperatures and high engine load, or very cold temperatures."  This research was criticized by some because most engine damage appears to be caused by cold starts, and their research method may not have included enough cold starts to be representative of personal vehicle use.
Synthetic base stocks, inside the engine oil formulations, decrease the emissions, increase the catalytic converter life with their low ash & zero sulphur properties and especially their low evaporation rates (Noack volatility values)
This section was taken from Castrol.com
Looking for the real story on synthetics? Wondering what's true and what's not? These are some misconceptions about synthetics, and facts to set the record straight:
- MYTH: Once I use synthetic motor oil in my car, I cannot switch back to conventional motor oil.
FACT: Synthetic motor oils are compatible with conventional and part synthetic (blend) motor oils. However, synthetics deliver superior engine protection versus conventional oils.
- MYTH: Using synthetic motor oil will void my vehicle’s warranty.
FACT: Using synthetic motor oil will not void your vehicle’s warranty. Synthetics are formulated to not only meet minimum required industry standards, but also, in many cases, meet tougher industry and vehicle manufacturer standards.
- MYTH: Synthetic oil is only for new cars.
FACT: Quality synthetic motor oil can be used in old as well as new cars, including vehicles in which conventional oil was previously used. Synthetic motor oils are beneficial for the good health, long life and top performance of new and old cars.
- MYTH: You need to allow a break-in period for new vehicles before using synthetic motor oil.
FACT: In the past, it was recommended that conventional motor oil be used for your first oil change to allow for some controlled wear to break-in the new engine. However, with current engine technology, a break-in period is not necessary. You can use synthetics immediately.
Let's Test - From hotrod.com by Steve Magnante
To see the difference between mineral-based and synthetic fluids, we enlisted the help of Scott Crouse's '65 Mercury Comet. With a hot 347, World Class T5 manual transmission, and 4.11 geared 9-inch rear axle, it's a rolling torture chamber for vital fluids. First we made a series of runs on the Westech Performance Superflow chassis dyno with 20W50 in the crankcase, Dexron III in the gearbox, and 75W90 gear oil in the TracLoc differential; the result was 408.3 horsepower and 405.1 lb-ft of torque. Then we drained the petro-chemicals and replaced them with man-made hydrocarbons from Royal Purple: 7 quarts of 20W50 synthetic engine oil, 6 quarts of Max ATF, and 2˝ quarts of Max Gear 75W90. After a 5-mile jaunt to get everything up to the same temperature as the baseline test, we let it rip. The monitor read 418.4 hp and 411.2 lb-ft of torque, a gain of 10.1 hp and 6.1 lb-ft just by switching to synthetics: an impressive tribute to the reduced coefficient of friction. We've seen similar improvements on the engine dyno, and have noted reduced wear through the use of synthetics. They're a bunch more expensive, but in our opinion, they're worth it for cars you care about. For your $200 Pinto, stick with the 99-cent stuff.