"We have noticed that the viscosity of the oil in our circulating oil system has been decreasing. What could be the reason for this?"
There can be a variety of reasons why viscosity may drop or even drop suddenly without warning. However, the most common cause is the addition of a lower viscosity fluid than required. For example, if you are using an ISO 220 viscosity gear oil but add a lower viscosity gear oil of the same brand (such as ISO 68), this may cause a sudden drop in the viscosity reading when the oil is sampled.
Viscosity can also decrease when non-lubricants such as solvents and diesel fuel are accidentally introduced into the lubricant. If this happens, it's best to change the oil.
Another way a lubricant can lose viscosity is through loss of viscosity index (VI) improver or shearing. For example, if you use a multigrade SAE gear or engine oil (such as 10W-30), the oil contains an additive called a viscosity index improver. During use, the VI improver can slough off and break down, causing the oil's viscosity to decrease. Remember, exposure to high temperatures is the biggest factor in complete failure of viscosity index improvers.
Multigrade oils are not the only oils that can become thinner due to high temperatures. Oil running at extremely high temperatures can begin to thermally crack. High temperatures can shear/crack oil molecules into smaller molecules, resulting in a decrease in viscosity. If you have problems with mineral oil losing viscosity at high temperatures, consider switching to a synthetic oil for your application.
Remember, viscosity can rise, fall, or stay the same. The list of root causes that can change viscosity readings is extensive. This is why viscosity has become an informative indicator of the condition of used oil. After all, when the viscosity does not change, you can rightly conclude that many of the known viscosity changing factors may not be occurring. The bad thing is when the viscosity shifts suddenly with no obvious explanation or warning. Then you have to determine what this means and why it happened.
It is safe to say that viscosity will not change without a forcing event or condition that causes the change. While the petroleum analysis community is aware of common suspicious situations or events, there are still some that remain undiscovered or at least not fully understood.
