Importance of oil viscosity

Viscosity is considered the most important property of a lubricant because it determines the lubricant's film strength and efficiency in preventing friction between moving parts.

Oil with higher viscosity can withstand greater pressure without being squeezed out of the lubricating surfaces. However, the high internal friction of the oil may offer greater resistance to the movement of the lubricating parts. An oil of lower viscosity offers less resistance to the moving parts, but the oil can be easily squeezed out of the lubricating surfaces.

Viscosity determines film thickness and film strength in machines. Minimum, maximum and optimum viscosity requirements demanded by machines do not take temperature into account. Instead, a machine’s viscosity requirement is based on such things as component design (i.e. bearing), loads and speeds.

Using oil with the incorrect viscosity, whether too high or too low, can cause a range of issues with equipment.

Too little viscosity can cause:

  • poor lubrication
  • excessive heat
  • increased wear and friction

Too much viscosity can cause:

  • poor flow
  • energy waste
  • start-up problems

Viscosity Index

The viscosity index (VI) is the rate of change of viscosity between two temperatures. The lower the VI, the more the drop in viscosity as the oil warms up. The higher the VI value, the less the drop in viscosity as the oil warms up.

Viscosity changes with temperature therefore the measuring temperature must be specified whenever the viscosity of a liquid is stated. When temperature rises, a liquid becomes less viscous. Similarly, a liquid becomes thicker when temperature drops.
Viscosity values of lubricants are typically provided at 40 and 100 degrees Celsius.

Viscosity increase or decrease

Assuming that no water is emulsified in the system reasons for a viscosity increase may be due to the oil having oxidized, an increase in pressure or decrease in temperature, contamination with a higher viscosity fluid, or evaporative losses of light oil fractions from high temperatures, glycol contamination and soot contamination.
It normally increases slowly with use. An increase of more than 25% usually indicates that the oil should be replaced. If the increase is sudden, a mechanical problem may be present in the compressor, or the oil may have been contaminated.

The majority of the characteristics associated with wrong, contaminated or degraded lubricants will cause a change in viscosity. These include oxidation, shear thinning, thermal degradation and many other common damaging conditions.

Any significant change of oil viscosity, either a reduction or increment, may damage lubrication film stability and effectiveness as shown in the table below:

Reduction in viscosity Increase in viscosity
Thermal cracking of oil molecules Oxidation
Share thinning of VI impovers Water (emulsion)
Cross mixing with lower viscosity oil Cross mixing with higher viscosity oil
  Antifreeze (glycol)

Selecting a lubricating oil of appropriate viscosity is essential to achieve optimum lubrication of equipment. For help and advice on choosing the right viscosity for your machinery please contact your nearest Anderol® distributor.