Viscosity affects heat generation in bearings, cylinders and gear sets related to an oil's internal friction. It governs the sealing effect of oils and the rate of oil consumption, as well as determines the ease with which machines may be started or operated under varying temperature conditions, particularly in cold climates.

Viscosity is a measure of an oil's resistance to flow. It decreases (thins) with increasing temperature and increases (or thickens) with decreased temperature. These conditions explain why an oil will flow much more easily in summer at a temperature of 25 degrees C (78 degrees F), than it will flow in winter at minus 25 degrees C (minus 13 degrees F).

An oil's viscosity is measured most commonly by kinematic viscosity and reported in a unit called the centistoke (cSt). Kinematic viscosity is measured in the time it takes for a specific volume of oil to flow through a special device called a capillary tube.

Not all oils respond in the same way to a given change in temperature. Many oils contain an ability to resist changes in viscosity due to a change in temperature. This property is referred to as the oil's viscosity index or VI. The higher the VI of an oil, the less its viscosity is altered by temperature changes.

The benefits of oils with a higher VI are:

  1. A general increase in viscosity at higher temperatures, which results in lower oil consumption and less wear.
  2. A reduced viscosity at lower temperatures, which will improve starting and lower fuel consumption.

Another factor in the measurement of viscosity is the ability of an oil to resist shearing or the "tearing away of one plane of lubricant from another" during the hydrodynamic lubrication function.

However, under certain conditions, such as shock loads, continuous heavy loading, extremely high temperatures and/or critically low (thin) viscosity, lubricants may not remain in their normal hydrodynamic film state.

A condition begins where there is intermittent contact between the wear surfaces. This intermittent contact is called boundary lubrication, and damage starts to occur. If the conditions noted above are not corrected immediately and boundary lubrication continues, a failure due to the lack of an oil film can occur within hours.

Kinematic viscosity, viscosity index and shear stress/shear rate are all factors that should be taken into account by a lubricant manufacturer when blending lubricating oils, but what does all this mean to the end user? It means that the viscosity of an oil is the first and most important consideration when selecting an oil for a specific application.

Remember, for the most effective lubrication, the viscosity must conform to the speed, load and temperature conditions of the lubricated parts.