The three primary types of motor oil are conventional, synthetic and synthetic blend. There also are different grades, or weights, within these types. Weights of motor oil are based on the viscosity index, a measure of the resistance of liquid to flow. Viscosity of different grades of oil reacts differently to lower and higher temperatures.
Conventional motor oil is a byproduct of the crude-oil refining process and is the least expensive of the three types.
Synthetic motor oil flows better than conventional motor oil, keeps the engine cleaner and maintains viscosity longer. It is the most expensive motor oil.
Synthetic blends provide some benefits of synthetic oil while not costing as much.
Single grade is the designation from the Society of Automotive Engineers, such as SAE30, a measurement of oil thickness and ability to handle high temperatures.
Multigrade, for instance, 5W30, indicates the oil's ability to handle both hot and cold temperatures. In this measurement, "W" stands for "winter," and the lower number represents the lowest temperature at which this oil performs well.
