How Important is Oil Viscosity?

The Fundamentals of Viscosity

The most important single property of a lubricant is its viscosity.  It is defined as a measure of a lubricant's resistance to flow.  The viscosity of any fluid changes inversely with temperature.  As temperature increases, viscosity decreases and as temperature decreases, viscosity increases.  The most common way that viscosity is measured is Kinematic Viscosity, reported in centistokes (cSt).  An oil’s viscosity is then classified according to one of a number of different classifications.

Monitoring and trending viscosity is perhaps one of the most important components of any oil analysis program. Even small changes in viscosity can be magnified at operating temperatures to the extent that an oil is no longer able to provide adequate lubrication. Typical industrial oil limits are set at ±5 percent for caution, and ±10 percent for critical, although severe-duty applications and extremely critical systems should have even tighter targets.

A significant reduction in viscosity can result in:

•  Loss of oil film causing excessive wear
•  Increased mechanical friction causing excessive energy consumption n Heat generation due to mechanical friction n Internal or external leakage
•  Increased sensitivity to particle contamination due to reduced oil film
•  Oil film failure at high temperatures, high loads, or during start-ups or coast-downs.

Likewise, too high a viscosity can cause:

•  Excessive heat generation resulting in oil oxidation, sludge, and varnish build-up
•  Gaseous cavitation due to inadequate oil flow to pumps and bearings
•  Lubrication starvation due to inadequate oil flow
•  Oil whip in journal bearings
•  Excess energy consumption to overcome fluid friction
•  Poor air detrainment or demulsibility
•  Poor cold-start pumpability

Whenever a significant change in viscosity is observed, the root cause of the problem should always be investigated and corrected. Changes in viscosity can be the result of a change in the base oil chemistry (a change in the oil’s molecular structure), or due to an ingressed contaminant.

Viscosity changes may require additional tests, such as acid number (AN) or Fourier transform infrared spectroscopy (FTIR), to confirm incipient oxidation; contaminant testing to identify signs of water, soot or glycol ingress; or other less commonly used tests, such as the ultracentrifuge test or gas chromatography (GC), to identify a change in the base oil chemistry.

Viscosity is an important physical property that must be monitored and controlled carefully because of its impact on the oil and the oil’s impact on equipment life.  Whether measuring viscosity onsite using one of many onsite oil analysis instruments capable of determining viscosity changes accurately, or whether sending samples routinely to an outside lab, it is important to learn how viscosity is determined, and how changes can impact equipment reliability. A proactive approach must be taken to determine the condition of the equipment’s lifeblood - the oil!

About TestOil   

With more than 30 years of experience in the oil analysis industry, TestOil focuses exclusively on assisting industrial facilities with reducing maintenance costs and avoiding unexpected downtime through oil analysis program implementation. As industry experts in diagnosing oil-related issues in equipment such as turbines, hydraulics, gearboxes, pumps, compressors and diesel generators, TestOil provides customers with a guarantee of same-day turnaround on all routine testing. With in-house certified training professionals, TestOil offers lubrication and oil analysis training, private onsite training, certification training and exams, and educational webinars. For more information on partnering with TestOil on oil analysis programs or training opportunities visit   Contact us: 216-251-2510;

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