Had recently condemned 5000ltr system lub oil (Mineral oil, 2000hrs) due to high viscosity, other properties of the lub oil was well within the limits.

Marin diesel 4S trunk engines are running on IFO-40 with 2% sulfur, Tropical environment and mostly at low load, lub oil purifiers are running continuously and engine is equipped with lub oil centrifuges.

I've asked the oil company for cause, but received just a standard answer. (Newer add more that 10% when topping up system, make sure purifiers are running, etc..)

Here are some causes present in my mind (Oxidation, additives that evaporates, additives that are washed away (Reaction with water in purifier), polymerized)

Assume that all other properties are within the limits

Thanks for a good forum

This message has been edited. Last edited by: Espen,

Espen,
Have you checked the water content? Higher water content leads to increase in viscosity. If the centrifuge is not working well, there are chances of water contamination.

What are your alarm limits for water content and viscosity level?

High viscosity may develop for different reasons but mainly for water and oxidation. A change in viscosity up to 10% is accepted. more than 10% and up to 20% you don't need to condemn the oil, but to pay attention to what is causing the change. Compare TBN value with the new oil, it may increase up to 1 mgKOH/g that is a sign of oxidation too. If water content is less than 0.05 and TBN normal, and if the coloration is OK (meaning not dark brown), and the oil is not cloudy (meaning water turbidity) then it might be a wrong analysis, take a sample and send it again for validating the previous result.