Viscosity - Lubricating Oil Analysis
Viscosity is one of the most important properties of lubricating oil. Viscosity is a measurement of resistance to flow at a specific temperature in relation to time. The two most common temperatures for lubricating oil viscosity are 40°C and 100°C. Viscosity is normally evaluated with a kinematic method and reported in centistokes (cSt). In used oil analysis, the used oil's viscosity is compared to that of the new oil to determine whether excessive thinning or thickening has occurred.
Viscosity Index (VI) is the change in flow rate of a lubricant with respect to temperature. Oil with a high VI resists thinning at high temperatures. Use of high VI oil is recommended in engines and other systems that operate at elevated temperatures.
Cause
High Viscosity
• Contamination soot/solids
• Incomplete combustion-A/F ratio
• Oxidation degradation
• Leaking head gaskets
• Extended oil drain interval
• High operating temperature
• Improper oil grade
Low Viscosity
• Additive shear
• Fuel dilution
• Improper oil grade
Effect
High Viscosity
• Increased operating costs
• Engine overheating
• Restricted oil flow
• Accelerated wear
• Oil filter by-passed
• Harmful deposits/sludge
Low Viscosity
• Engine overheating
• Poor lubrication
• Metal-to-metal contact
• Increased operating costs
Solution
• Check air-to-fuel ratio
• Check for incorrect oil grade
• Inspect internal seals
• Check operating temperature
• Check with lube supplier for advice
• Check for leaking injectors
• Evaluate equipment use vs. design
• Evaluate operating conditions
• Use trained operators
• Change oil and filters
• Check for loose fuel crossover lines
