While it is usual for your oil to reduce in viscosity as it ages, there are a number of reasons that you might find your lubricating oil thickening. Thicker oil can lead to higher operating temperatures, increased drag and decreased efficiency.

The main reason for oil thickening is topping it up with another oil of a higher viscosity. We often come across cases where lubricators have used whatever product was available to top up a piece of equipment, but this will affect the thickness if it’s a different viscosity. You should always top up a system with the same product, or another with similar properties which is fully miscible.

Oil degradation and contamination can also lead to an increase in viscosity. Contamination with dirt can increase the viscosity if there are significant levels within the oil. Water contamination can also lead to an increase in viscosity as the oil emulsifies. Using regular oil analysis can help identify issues of contamination and oxidation, allowing you to take preventative action.

Another reason for increasing oil viscosity is evaporation. This is most common in poorly formulated oils, and occurs as the oils are subjected to high temperatures. Where this occurs, it’s best to consider changing the oil that you’re using in the application.

If you are using regular oil analysis, you should be able to identify any increase in viscosity, and hopefully the root cause of the issue. Routinely monitoring your lubricants allows you to trend features such as oxidation, so you will know when an unusual problem arises.