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Viscosity is one of the most important indicators of lubricant health but is too often disregarded as a condemning parameter worth taking seriously.
Consequences of high viscosity
Foaming, increased energy consumption, lubricant starvation due to lack of flow (this is compounded at low temperatures – especially at start up), cavitation, and filter by-pass.
Causes of high viscosity
The most common cause of high viscosity is the wrong oil or make-up oil added to the system. Other causes include oxidation, nitration, soot, water, or contamination with an incompatible fluid
Corrective Action
If the high viscosity is caused by soot or water, they can sometimes be filtered out. If the correct fluid but the wrong viscosity grade was added to a system, it is sometimes possible do remove some of the oil and add a lower viscosity grade of the same product to blend to the correct viscosity. (This works best with non-engine oils and straight grade oils.) More often than not, however, an oil change is required.
Consequences of low viscosity
Increased wear due to lack of hydrodynamic lubrication, higher temperatures (increased thermal degradation as a result), poor pump performance, and increased oil consumption in engines
Causes of low viscosity
Wrong oil or wrong make-up oil, fuel contamination in engines, shearing of viscosity improver additives or contamination with aromatics such as solvents.
Corrective Action
If the correct fluid but the wrong viscosity grade was added to a system, it is sometimes possible to remove some of the oil and add a high viscosity grade of the same product to blend to the correct viscosity. In most cases, however, the corrective action indicated is to change the oil.