Simply stated, viscosity is defined as the internal resistance of a fluid to flow. 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.

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. 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.

In recent years, some large lubricant marketers have run advertisements on TV that highlight the importance of viscosity breakdown. 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. Simply stated, viscosity is a measure of a fluid’s internal resistance to flow. 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. The resistance to flow, or viscosity, of the second oil had increased dramatically with the decrease in temperature. This ad illustrated just how important it is to consider viscosity when choosing the proper lubricant for a specific application.

Lubricant viscosity or consistency is a physical measurement of a lubricant’s internal resistance to flow. It can truly affect how the lubricant will function in a piece of equipment. Fortunately, organizations like ASTM, SAE, AGMA, ISO and others have created standards for lubricant viscosity and consistency that are to be used as guidelines when selecting the proper lubricant. If the wrong lubricant viscosity is selected for an application, the chances for equipment failure are dramatically increased. Therefore, the best rule is to always check the original equipment manufacturer’s manual for lubricant viscosity recommendations.

If the OEM makes no recommendations, then the next step is to consider the operating speed, temperature and load of the application to be lubricated. Finally, after making a lubricating product selection, it is important to closely monitor the equipment to ensure the right choice was made. If possible, visually observe the moving parts to verify that a sufficient lubricant film is present to protect them. If not, listen for any unusual load grinding, chattering or squalling noises, which often are indications of metal-to-metal contact. Finally, one last technique is to contact lubricant manufacturers for recommendations. They often can provide technical support for proper fluid or grease selection.

Very simplistically, viscosity is a measure of a lubricant’s physical resistance to flow, yet it is probably the most important property of a lubricant.