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Synthetic Motor Oils
William D. Siuru, Jr., PhD, PE
Seeing the ads for synthetic motor oils, you may ask yourself, "is this something that I need for my engine?" And, "are they are worth the extra money?"
Conventional mineral motor oils are refined using crude oil for the base stock. In contrast, the base stocks for synthetic oils are man-made organic esters and other synthesized hydrocarbons. Produced from crude petroleum, minerals oils contain impurities like nitrogen, resin, asphalt, wax, carbon, sulfur and aromatic residues. Refining removes most impurities, but some still remain to cause problems especially under high stress and temperatures. Synthetic oils are developed in the laboratory to provide the exact characteristics desired. These "designer" oils include no impurities at least when poured from the can. Impurities can, of course, appear during combustion.
Synthetic oils can tolerate much higher operating temperatures, for example when towing or hill climbing in warm temperatures with the air conditioning operating. Even the best mineral-based motor oils breakdown or oxidize at temperatures of 250 degrees F to 300 degrees F. Above this, the life of these oils drops drastically resulting in increased engine wear and formation of harmful sludge, varnish and other deposits. Synthetic oils can easily tolerate temperatures up to 450 degrees F and some can go up to 700 degrees F. With synthetic oils there are no impurities to start with so they can resist oxidation up to ten times longer.
An oil's viscosity index (VI) describes how the oil's viscosity changes with temperature. The higher VI, the greater the viscosity stability and the ability of an oil to retain its viscosity by resisting shearing at high temperatures and engine speeds. Shearing reduces an oil's ability to lubricate by tearing away and thinning out an oil's film that prevents metal-to-metal contact. Synthetic oils have a much higher VI and thus viscosity stability.
Synthetic oils also work better at low temperatures because they flow much more freely right down to -40 degrees F to -50 degrees F. Wax, not found in synthetic products, that congeals at low temperatures is the culprit here. At very low temperatures, mineral oils can become so thick they will not flow at all. Naturally, this makes for much easier winter starting. Also synthetic oils start lubricating much sooner and reaches the upper valvetrain much faster. These first few moments of operation are by far the toughest wear engines see.
Synthetic motors oils are much less volatile and are thus likely to be converted from liquid to vapor. Petroleum motor oils can lose as much as 25% of their weight when the light hydrocarbons are vaporized. Synthetic motor oils remain stable at high temperatures, losing perhaps only between 5% to 10% of their weight to vaporization. This mean lower oil consumption as well as reduced exhaust emissions. Because synthetic oils are more slippery, friction is reduced. Howev...