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

SAE Grade	cSt @ 100C min	cSt @ 100C max
10W	4.1	5.6
20W	5.6	9.3
30	9.3	12.5
40	12.5	16.3
75W	4.1	NA
80W	7.0	NA
85W	11.0	NA
90	13.5	24.0
140	24.0	41.0

ISO / AGMA Viscosity Range

ISO #	AGMA #	cSt @ 40C min	cSt @ 40C max
5	NA	4.14	5.06
7	NA	6.12	7.48
10	NA	9	11
15	NA	13.5	16.5
22	NA	19.8	24.2
32	NA	28.8	35.2
46	1	41.4	50.6
68	2	61.2	74.8
100	3	90	110
150	4	135	165
220	5	198	242
320	6	288	352
460	7 Comp	414	506
680	8 Comp	612	748

NA = Not Applicable