There are excellent synthetic and fuel-efficient oils available that, under the right circumstances, can help provide better fuel mileage and better engine protection. However, these advantages come at a price, which can be significantly more than the price per quart of conventional motor oils.
Before pouring any synthetic oils into your vehicle's engine, you should consider the condition of the engine and the type of driving you do.
Generally, it is best to avoid the use of synthetic oil in both brand new and older, high mileage engines. New engine require a proper break-in, and some sources feel that the synthetics are so slippery, that they can impede the normal wear engines need for break in. Some manufacturers recommend that you accumulate at least 5,000 miles (8,000 km) before switching to a synthetic oil. Conversely, older engines are looser and tend to lose more oil; synthetics will slip past worn parts more readily than regular oil. If you engine already leaks oils (due to worn parts or bad seals/gaskets), it may leak more with a synthetic oil.
All of the vehicles covered herein are designed to run on unleaded fuel. The use of a leaded fuel in a vehicle requiring unleaded will plug the catalytic converter and render it inoperative. It will also increase exhaust backpressure because the material in the converter will tend to collapse and plug the exhaust, severely reducing engine power. The minimum octane rating of the unleaded fuel being used must be at least 87, which usually means regular unleaded, but some high performance engines may require higher octane ratings. Fuel should be selected for the brand and octane which performs best with your engine.
The use of a fuel too low in octane (a measure of anti-knock quality) will result in spark knock. While the computer controlled ignition system uses knock sensors to detect knock and then electronically adjust the ignition timing to compensate, there is a limit to the system's adjustment capability. Since many factors such as altitude, terrain, air temperature and humidity affect operating efficiency, some light knocking (usually on acceleration) may result and is even considered normal. But if persistent knocking occurs, it may be necessary to switch to a higher grade of fuel. Continuous or heavy knocking may result in engine damage.