For the ultimate in high temperature protection, durability and all-round performance, synthetic oil is the way to go. Unfortunately, most synthetic motor oil costs about two to three times as much as conventional petroleum-based oils. They cost more because synthetic oil requires more processing and refining. Some synthetic POA oils are made from other chemical stocks rather than refined from petroleum. This improves the oil's performance in virtually every aspect.
Synthetic oil has superior temperature resistance. Synthetic oil can safely handle higher operating temperatures without oxidizing (burning) or breaking down. The upper limit for most mineral based oils is about 250 to 300 degrees F. Synthetic oil can take up to 450 degrees F or higher. This makes synthetic oil well-suited for turbocharged applications as well as high rpm and high output engine applications.
Synthetic oil has better low temperature performance. Synthetic oil flows freely at subzero temperatures, pouring easily at -40 or -50 degrees F. where ordinary oils turn to molasses. This makes for easier cold starts and provides faster upper valvetrain lubrication during the first critical moments when most engine wear occurs.
Synthetic oil improves engine performance. Synthetic oil tends to be more slippery than ordinary motor oil, which improves fuel economy, cuts frictional horsepower losses and helps the engine run cooler. The difference isn't great but it can make a noticeable improvement.
Synthetic oil allows longer oil change intervals. Because synthetic oil resists oxidation and viscosity breakdown better than ordinary motor oil, it does not break down or sludge up as fast as ordinary motor oil. Some synthetic oil suppliers say oil change intervals can be safely extended to 10,000 miles or longer (some as claim as much as 25,000 miles!).
CAUTION: For vehicles under warranty, extending the normal change interval beyond the vehicle manufactuer service intervals is NOT recommended because Not following the OEM maintenance schedule can void your powertrain warranty.
Synthetic oils are available in the same grades as ordinary motor oils (5W-30, 5W-20 & 10W-30) as well as "extended" grades such as 15W-50 and even 5W-50 and cold weather or ultra low viscosity EURO grades such as 0W-20 and 0W-40.
Who should use a synthetic oil? The premium-priced oil is best for:
A lower cost alternative to a full synthetic oil is a synthetic blend motor oil. A blend typically mixes about 25% synthetic oil with 75% conventional mineral oil. Many of today's 5W-20 multi-viscosity oils are actually synthetic blends. A blended product offers some of the same advantages of a full synthetic for less cost, but you do not get all of the advantages of a full synthetic oil because of the conventional oil content.
You can create your own "blend" by simply substituting a quart or two of a full synthetic oil for convention oil when you change oil. Synthetic oils are compatible with conventional motor oils.
Update Oct 30, 2007: A new green synthetic motor oil made from beef tallow (animal fat) is now available as an alternative to petroleum-based oils. The new oil is 100% biodegradeable, non-toxic, and safe (you can even drink it!). Performance is similar to other synthetic oils but it costs a little less. The product is called G-OIL.
Update: November 2010: For 2011, General Motors has announced a new oil requirement called "dexos." GM says their new oil performance specification is better than the new GF-5 specification, which also goes into effect this fall. GM says dexos is required in all 2011 and newer GM engines, and is backwards compatible with older engines that use SM oils.
There are two versions of dexos: dexos1 for gasoline engines and dexos2 for diesel engines. The specification requires a high quality synthetic base stock with additives that provide high temperature, high sheer characteristics to reduce friction for better fuel economy, additives to reduce piston ring deposits and sludge, and additives to extend oil life (necessary for use with GM's Oil Life Reminder System).
Because it uses high quality synthetic base stocks, dexos and other brands f oil that meet GM's dexos specification are premium-priced products. Most major brands of oil have a 5W-30 oil that meets the dexos specifications.
Update April 21, 2011: Royal Purple is introducing a special synthetic motor oil with extra anti-wear zinc/phosphorus (ZDDP) additive for high performance street engines that are out of warranty. The HPS motor oil is formulated for engines with high-lift cams and flat tappet cams that have increased loads on the camshaft lobes. Tests show the extra film strength and anti-wear additives in their HPS oil improves wear resistance compared to other synthetic and conventional motor oils. The new oil is available in the following multi-viscosity grades: 5W-20, 5W-30, 10W-30, 10W-40 and 20W-50.
Synthetic Motor Oil Sources: