Viscosity is a measurement of an oil's resistance to flow at a specific temperature and is a key indicator of lubricant serviceability.
If oil is too thick, or too thin, it will not function correctly and over time, if this condition remains undetected, it will lead to premature wear or can even lead to complete component failure.

Causes Effects
High Viscosity
Contamination soot/solids Extensive Wear
Oxidative degradation Restricted oil flow
Water ingress Impeded low temperature operation
Over extended oil drain Increased friction
High operating temperature Metal to metal contact
Increased oil grade or type Increase lubricant leakage
Low Viscosity
Polymer shearing (multigrade oils)  
Fuel dilution  
Incorrect oil grade or type  
Cross contamination  
Check fuel pumps and injector calibration, filters
Check for proper operating tempatures
Check operating conditions
Change oil and filters
Evaluate equipment suitablility for application

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