Dollars And Sense: Why You Can't Afford Not To Use SyntheticsExperts will point out that synthetics can typically perform adequately for twice the typical recommended oil change intervals, as long as you replace the filter at specified intervals. But GMHTP recommends that performance enthusiasts think of synthetic oil as a kind of insurance policy which stays in force when you change it frequently. While it's true that fresh synthetic can maintain acceptable integrity for 25,000 miles or a full year of use in a healthy engine, we think that's a bit like buying expensive running shoes to protect your feet and joints-and then over-using them to the point at which they're no better than junk shoes.
No matter how good a synthetic oil may be when fresh, any engine oil will gradually accumulate combustion blow-by and other contaminants that cannot entirely be filtered out and thus gradually damage the oil. What's more, any synthetic oil can degrade more quickly if abused.
Even if you're using a top-quality synthetic, we recommend you follow vehicle manufacturers' recommendations for "Severe Duty" applications. Synthetic oil does cost more. But how much? Consider 24,000 miles of usage. Assume you change conventional API SJ mineral oil yourself every 3,000 miles (good insurance), and that your engine requires 5 quarts of oil at $1.50 each per change, and that the filter costs $3.50, plus tax, for a total of, say, $11.50. Over two years, you'll need 7 oil changes, for a grand total of about $80. Now assume you use $4.50/qt. synthetic-just to be sure. The seven oil changes now cost about $195-a difference of $115 you could think of as a sort of "catastrophic-illness insurance" which amounts to less than $5 a month over two years. Clearly, this is less expensive than the hundreds or thousands of dollars it costs to replace internal engine parts.
The Soul Of A Synthetic: AdditivesOil additives are expensive but vital to the performance of any modern engine oil. Manufacturers exercise extreme care in order to ensure that no mutually antagonistic effects occur between additives themselves or with the base lubricant. In addition to its primary thick-film lubrication function, oil must control engine deposits and keep foreign particulate matter in suspension that is too small to be filtered from the oil. Oil must also prevent corrosion and provide thin-film lubrication when hydrodynamic lubrication fails and metal-to-metal contact does occur. Anti-wear additives form an inorganic thin film between engine metal surfaces which shears more easily than the base metal, allowing parts to slide against each other without causing component failure. Such components typically do not work well in cold oil, which is one reason starting a cold engine causes a lot of wear. The anti-wear additives eventually get used up, one more reason oil must be changed periodically.
Seventy percent of lubrication-related friction is caused by oil film molecules rubbing against each other. The other 30 percent comes from parts protected by thin film lubrication rubbing against each other. Friction Modifier additives further smooth this contact, providing a small additional reduction of friction.
A vital task of a modern oil is to keep sludge, varnish, and carbon from clogging up the works of an engine. Manufacturers add detergents and dispersants (mineral oils require more to combat innate contaminants) to keep small bits of trash too small to be filtered out suspended so they circulate until the next oil change rather than "coagulating" on parts in deposits large enough to have an abrasive effect. If the detergents and dispersants wear out-or sufficient crud accumulates that the oil can no longer hold it, sludge and varnish will coat the internal surfaces of an engine in a hurry. All modern engine oils contain additives designed to prevent oxidation of the base stock, and to neutralize acids formed from oxidized oil or combustion blow-by gases. Thin-film additives also help protect bearing surfaces from corrosion.