The presence of viscosity information on almost all lubricant marketers’ technical literature is an indication that it also is important in the marketing of lubricants. This ad illustrated just how important it is to consider viscosity when choosing the proper lubricant for a specific application. The resistance to flow, or viscosity, of the second oil had increased dramatically with the decrease in temperature. A good example of this was provided in one of the TV ads, which showed two oils being cooled until one continued to flow out of the bottle readily, while the second dropped out in blobs. Simply stated, viscosity is a measure of a fluid’s internal resistance to flow. These advertisements make it seem like viscosity is a complex chemical property of the fluid, when in fact it is a measurement of a physical property.

In recent years, some large lubricant marketers have run advertisements on TV that highlight the importance of viscosity breakdown. This paper will summarize some of the more commonly used viscosity standards, describe the tests used to measure viscosity, and eliminate some of the confusion all of these standards may create for the end user. Some examples are SUS, cSt, cP, ISO, SAE engine, SAE gear and AGMA it’s enough to make a person’s head start to spin. That doesn’t sound too difficult, does it? Unfortunately, new temperature, speed and pressure demands on lubricating fluids have changed over the years, resulting in several different measurements and classifications being created to describe lubricant viscosity. Simply stated, viscosity is defined as the internal resistance of a fluid to flow. By John Sander, VP Technology, Lubrication Engineers Inc.

Kinematic viscosity testing