Modern motor oils are complex blends of distilled petroleum products and special additives that help keep your engine running efficiently and cleanly. The main concepts used to rate motor oils are viscosity and types of additives. Mineral and synthetic oils behave similarly, but synthetic oils have better heat-carrying characteristics, improving engine wear.
Chose the correct SAE viscosity. "Viscosity" refers to the liquid property of the oil -- whether it's thin like milk or thick like pancake syrup. Since oils have to be pumped through an engine, an "SAE" rating has been developed to indicate how much flow the oil has at a standarized engine operating temperature of 100 C (215 F) degrees. Your car manual will suggest the proper SAE engine oil rating.
Choose the correct winter or summer oil. Butter melts at a high temperature and congeals at a low temperature. Likewise, certain motor oils congeal at low temperatures, making a cold engine hard to crank. A motor oil with a "W" rating has been tested to have a low viscosity at cold temperatures and a higher viscosity at engine operating temperature. So a winter oil rated 5W-30 at a cold temperature has the viscosity of SAE 5, and at normal engine temperature has the viscosity of SAE 30.
Engine oils also vary in the types of additives manufacturers add. All modern oils contain additives such as anti-foaming agents, anti-corrosion agents, viscosity modifiers and detergents to improve performance. Detergent additives, for example, clean carbon soot created in internal combustion engines and suspend it in the oil so that it can eventually be removed from the engine. Oil has to be changed periodically because it becomes contaminated with "sludge" created by soot, water, acids, etc. that form as the engine runs but cause engine wear.
Understand the difference between mineral and synthetic oils. Synthetic oils are chemically engineered motor oils that have uniformly sized molecules. Contrary to popular belief, synthetic oils need to be changed just as frequently as mineral motor oil since all oils become contaminated with grit, water and acids during normal engine use. Synthetic oils, however, transfer heat more efficiently than natural oils and may improve engine life, especially in air-cooled engines.
Finally, engine age and the intended use of the engine can change the rating of an oil best used in a vehicle. In high mileage vehicles, using an oil with a slightly higher viscosity rating (for example, moving from 5W-30 to 10W-40) may reduce noise and improve oil pressure. Prolonged high load operation, such as towing a trailer across the country, might also call for a similar higher rated oil.
