SYNTHETIC OIL
There are many excellent synthetic and fuel-efficient oils currently available that can provide better gas mileage, longer service life, and in some cases better engine protection. These benefits do not come without a few hitches, however, the main one being the price of synthetic oils, which is three or four times the price per quart of conventional oil.
Synthetic oil is not for every car and every type of driving, so you should consider your engine's condition and your type of driving. Also, check your car's warranty conditions regarding the use of synthetic oils.
Both brand new engines and older, high mileage engines are the wrong candidates for synthetic oil. The synthetic oils are so slippery that they can prevent the proper break-in of new engines; most manufacturer's recommend that you wait until the engine is properly broken in - 5,000 miles (8,046km) - before using synthetic oil. Older engines with wear have a different problem with synthetics: they use (consume during operation) more oil as they age. Slippery synthetic oils get past these worn parts easily. If your engine is using conventional oil, it will use synthetics much faster. If your car is leaking oil past old seals you'll have a much greater leak problem with synthetics.
Consider your type of driving. If most of your accumulated mileage is high speed, highway type driving, the more expensive synthetic oils may be of benefit. Extended highway driving gives the engine a chance to warm up, accumulating less acids in the oil and putting less stress on the engine over the long run. Under these conditions, the oil change interval can be extended (as long as your oil filter can last the extended life of the oil) up to the advertised mileage claims of the synthetics. Cars with synthetic oils may show increased fuel economy in highway driving, due to less internal friction. However, many automotive experts agree that 50,000 miles (80,465km) is too long to keep any oil in your engine.
Cars used under harder circumstances, such as stop-and-go, city type driving, short trips, or extended idling, should be serviced more frequently. For the engines in these cars, the much greater cost of synthetic or fuel-efficient oils may not be worth the investment. Internal wear increases much quicker on these cars, causing greater oil consumption and leakage.
