Viscosity is the single most important property of a lubricant.
It is defined as the oil's resistance to flow or "thickness". Many different viscosity classifications systems are utilized, but all of them relate to the thickness of the oil. As the numbers increase within each system, the oil increases in viscosity.
Viscosity is the key test used to determine a number of used oil conditions.
When tested, a used oil sample viscosity is compared to its' original or new viscosity. Increases or decreases are measured. Increases indicate oil oxidation, addition of improper grade oil, fuel soot build up, dirt and antifreeze contamination. Decreases indicate fuel dilution, addition of improper grade oil and additive shear. One must be aware, however, that combinations of these conditions can cancel individual effects on viscosity, and for this reason, backup or support tests are needed to confirm specific problems.
Note: Oil additive shear is a factor related solely to multi-weight oils. Single grade oils do not contain viscosity index improvers and thus do not display this property.
SAE Viscosity Ranges
ISO / AGMA Viscosity Range
NA = Not Applicable