How does oil affect my engine's durability as well as its fuel economy?

Operating Viscosity

Of the different types of viscosity measurements, an oil's operating viscosity is the most important. It is this property which permits an engine to operate at all since it determines the oil's ability to lubricate critical areas of the engine such as the bearings, where temperatures can be greater than 300°F and the piston rings where temperatures will be much higher.

An oil of sufficiently high viscosity at high operating temperatures protects the engine parts from damaging contact with one another. However, even though higher operating viscosity is more protective, it also takes more energy to move the engine parts, resulting in poorer fuel economy. So there is reason not to use oils of viscosities higher than needed for protection.

Most multigrade oils experiencing the very high stresses in an engine also experience a loss in viscosity both temporarily and permanently as a result of encountering these stresses. An oil's resistance to viscosity loss during operation directly affects its ability to minimize engine wear. This is discussed in more detail under Shear Stability.

With the advent of multigrade oils, it has become necessary to measure the operating viscosity at the high flow rates and tight tolerances encountered between the moving surfaces in an engine. Thus, the operating viscosity of an oil is measured using ASTM high shear rate test methods such as D4683. In this test, a small amount of the oil is placed in a stator with a very close-fitting rotor (the rotor-stator gap is 1/100th of a fine human hair) and brought to the test temperature of 150°C (302°F). The rotor is then forced to turn at 3600 RPM with this narrow rotor-stator gap. The force required to turn the rotor at this speed and gap is measured and translated to viscosity in units of centiPoise (cP).

The SAE's engine oil classification system sets the following values for multigrade engine oils according to their high temperature, high shear rate viscosities (the number after the 'W'):

(Note: Centipoise is the amount of force necessary to move a layer of liquid in relation to another liquid. Centipoise is the standard unit of measurement for fluids of all types.)