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 | |
| Recommendations |
| Check fuel pumps and injector calibration, filters |
| Check for proper operating tempatures |
| Check operating conditions |
| Change oil and filters |
| Evaluate equipment suitablility for application |
