The SAE (Society of Automotive Engineers) grade number indicates the viscosity of the engine oil; its resistance to flow at a given temperature. The lower the SAE grade number, the lighter the oil. For example, the mono-grade oils begin with SAE 5 weight, which is a thin light oil, and continue in viscosity up to SAE 80 or 90 weight, which are heavy gear lubricants. These oils are also known as "straight weight", meaning they are of a single viscosity, and do not vary with engine temperature.
Multi-viscosity oils offer the important advantage of being adaptable to temperature extremes. These oils have designations such as 10W-40, 20W-50, etc. The "10W-40" means that in winter (the "W" in the designation) the oil acts like a thin 10 weight oil, allowing the engine to spin easily when cold and offering rapid lubrication. Once the engine has warmed up, however, the oil acts like a straight 40 weight, maintaining good lubrication and protection for the engine's internal components. A 20W-50 oil would therefore be slightly heavier than and not as ideal in cold weather as the 10W-40, but would offer better protection at higher rpm and temperatures because when warm it acts like a 50 weight oil. Whichever oil viscosity you choose when changing the oil, make sure you are anticipating the temperatures your engine will be operating in until the oil is changed again. Refer to the oil viscosity chart for oil recommendations according to temperature.
The API (American Petroleum Institute) designation indicates the classification of engine oil used under certain given operating conditions. Only oils designated for use "Service SG" should be used. Oils of the SG type perform a variety of functions inside the engine in addition to the basic function as a lubricant. Through a balanced system of metallic detergents and polymeric dispersants, the oil prevents the formation of high and low temperature deposits and also keeps sludge and particles of dirt in suspension. Acids, particularly sulfuric acid, as well as other by-products of combustion, are neutralized. Both the SAE grade number and the ape designation can be found on top of the oil can.
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 is 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 manufacturers recommend that you wait until the engine is properly broken in (3,000 miles) 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. Also, 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 a 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 (80,000 km) 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.
It is important to use fuel of the proper octane rating in your car. Octane rating is based on the quantity of anti-knock compounds added to the fuel and it determines the speed at which the gas will burn. The lower the octane rating, the faster it burns. The higher the octane, the slower the fuel will burn and a greater percentage of compounds in the fuel prevent spark ping (knock), detonation and preignition (dieseling).
As the temperature of the engine increases, the air/fuel mixture exhibits a tendency to ignite before the spark plug is fired. If fuel of an octane rating too low for the engine is used, this will allow combustion to occur before the piston has completed its compression stroke, thereby creating a very high pressure very rapidly.
Fuel of the proper octane rating, for the compression ratio and ignition timing of your car, will slow the combustion process sufficiently to allow the spark plug enough time to ignite the mixture completely and smoothly. Many non-catalyst models are designed to run on regular fuel. The use of some super-premium fuel is no substitution for a properly tuned and maintained engine. Chances are that if your engine exhibits any signs of spark ping, detonation or pre-ignition when using regular fuel, the ignition timing should be checked against specifications or the cylinder head should be removed for decarbonizing.
Vehicles equipped with catalytic converters must use unleaded gasoline only. Use of leaded fuel shortens the life of spark plugs, exhaust systems and EGR valves and can damage the catalytic converter. Most converter equipped models are designed to operate using unleaded gasoline with a minimum rating of 87 octane. Use of unleaded gas with octane ratings lower than 87 can cause persistent spark knock which could lead to engine damage.
Light spark knock may be noticed when accelerating or driving up hills. The slight knocking may be considered normal (with 87 octane fuel) because the maximum fuel economy is obtained under condition of occasional light spark knock. Gasoline with an octane rating higher than 87 may be used, but it is not necessary (in most cases) for proper operation.
If spark knock is constant, when using 87 octane fuel, at cruising speeds on level ground, ignition timing adjustment may be required.
If you plan to drive your car outside the United States or Canada, there is a possibility that fuels will be too low in anti-knock quality and could produce engine damage. It is wise to consult with local authorities upon arrival in a foreign country to determine the best fuels available.