What is Viscosity?

What is Viscosity?

Engine oil viscosity refers to how easily oil pours at a specified temperature. it must be considered when selecting a pump and determining the friction losses in a fluid process system.

Thin oils have lower viscosity and pour more easily at low temperatures than thicker oils. Thin oils reduce friction in engines and help engines start quickly during cold weather. 

Thick oils  have a higher viscosity.

Viscosity Dilution

The reduction of lubricant viscosity caused by contamination with water or other substances.

Viscosity Grade

Any of a number of systems, which characterize lubricants according to viscosity for particular applications, such as industrial oils, gear oils, automotive engine oils, automotive gear oils, and aircraft piston, engine oils.

Viscosity Index

The measure of the rate of change of viscosity with temperature. Heating tends to make lubricants thinner; cooling makes them thicker. The higher the viscosity index is on a particular fluid, the less of a change in viscosity there will be over a given temperature range. In determining the viscosity index, two temperatures of viscosity are taken, one at 40°C and the other at 100°C.

Viscosity Index Improver

Lubricant additive, usually a high molecular weight polymer, that reduces the tendency of an oil to change viscosity with temperature. Multigrade oils, which provide effective lubrication over a broad temperature range, usually contain viscosity index improvers.

Viscosity Modifier

Lubricant additive, usually a high molecular weight polymer, that reduces the tendency of an oil's viscosity to change with temperature.

Viscosity Absolute

The ration of the shearing stress to the shear rate of a fluid. It is usually expressed in centipoise.

Viscosity Kinematic

The absolute viscosity divided by the density of the fluid. It is usually expressed in centistokes.

Viscosity-Temperature Relationship

The manner in which the viscosity of a given fluid varies inversely with temperature. Because of the mathematical relationship that exists between these two variables, it is possible to predict graphically the viscosity of a petroleum fluid at any temperature within a limited range if the viscosities at two other temperatures are known. The charts used for this purpose are the ASTM Standard Viscosity-Temperature Charts for liquid Petroleum Products, available in 6 ranges. If two know viscosity-temperature points of a fluid are located on the chart and a straight line drawn through them, other viscosity-temperature values of the fluid will fall on this line; however, values near or below the cloud point of the oil may deviate from the straight-line relationship.

(Source: Machinery Lubrication)


Possessing viscosity. Frequently used to imply high viscosity.

Viscometer or Viscosimeter

An apparatus for determining the viscosity of a fluid.

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