Viscosity is one of the most important properties of lubricating oil. Viscosity is a measurement of resistance to flow at a specific temperature relative 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.
 

Water in an engine may be an indication of internal coolant leaks – a leaking head gasket, a cracked head or sleeves, or from external contamination - condensation of ambient atmospheric humidity, or from internal. Although water is typically evaporated by engines at normal operating temperatures, water may remain in the oil when engine temperatures are too low for evaporation to occur.
 
Oil analysis can effectively indicate the presence of water or coolant contamination before a major problem occurs. Infrared analysis is used to determine water content in used oil and these results are reported in percent volume. The Karl Fischer method is used to measure water in systems that are sensitive to low moisture content and these “KF” results are reported in ppm.
 

Fuel dilution of crankcase oil by unburned fuel can reduce lubricant viscosity and decrease its effectiveness. Thinning of the lubricant leads to decreased lube film strength and adds to the risk of abnormal wear. Depending on certain variables, when fuel dilution of crankcase oil exceeds 2.5 to 5%, corrective action should be taken. Fuel dilution is measured by gas chromatography with results reported in percent volume.
 

Solids Content represents all solid and solid-like constituents in the lubricant. The makeup of solids depends on the system. In diesel engines, fuel soot is usually the major constituent measured. In non-diesel components, wear debris and oil oxidation products are measured. All solid material is measured and reported as a percentage of sample volume or weight.
 

Fuel soot or carbon and is always found in diesel engine oil. Laboratory testing is used to determine the quantity of fuel soot in used oil samples. Stringent exhaust emission regulations have placed greater emphasis on fuel soot levels. One of the most significant impacts of reduced emissions is control of particulate emissions, which resulted in greater soot levels in the crankcase. The fuel soot level is a good indicator of engine combustion efficiency and should be monitored on a regular basis for possible maintenance action. It is interesting to note that engine soot is up to 85% as hard as diamond and if not kept within reasonable limits can have a very high abrasive result on the engine.
 

Lubricating oil in engines and other components combines with available oxygen under certain conditions to form harmful by-products. Heat, pressure, and catalyst materials accelerate the oxidation process. By-products of oxidation form lacquer deposits, corrode metal parts, and thicken oil beyond its ability to lubricate. Most lubricants contain additives that inhibit or retard the oxidation process.
 
Differential infrared analysis offers the only direct means of measuring the level of oxidation in oil. Note: A new oil reference is required for accurate measurement of oxidation. Results are reported on an absorbance scale.
 

Nitration products are formed during the fuel combustion process when combustion by-products enter the engine oil during normal operation or as a result of abnormal “blow-by” past the compression rings. These products, which are more common in oils used to lubricate natural gas- and propane- fueled engines, are highly acidic, create deposits, and accelerate oil oxidation. Infrared analysis represents the only method of accurately measuring nitration products in oil. Results are reported on an absorbance scale.
 

The total acid number is the quantity of acid or acid-like constituents in the lubricant. An increase in TAN from that of the new lubricant should be monitored. The TAN of new oil is not necessarily zero since oil additives can be acidic in nature. Increases in TAN usually indicate lube oxidation or contamination with water or an acidic product. TAN is an indicator of oil serviceability.
 

The total base number is an expression of the amount of alkaline additives in the lubricant that are capable of neutralizing the acid products of combustion. New oil starts with the highest TBN it will possess. During the time the lubricant is in service, the TBN decreases as the alkaline additives neutralize acids. TBN is an essential element in the establishment of oil drain intervals since it indicates whether the additives are still capable of providing sufficient engine protection. Most diesel engine manufacturers require the oil drained when its TBN reaches one-half or one-third its original value.
 

Fluid cleanliness is critical in hydraulic and other systems where high fluid pressure and velocity are involved. Excessive fluid particulate contamination is a major cause of failure of hydraulic pumps, motors, valves, pressure regulators, and fluid controls. Failure due to excessive particulate contamination is normally segregated into three areas:
 

Performance degradation
 
Intermittent failure
 
Catastrophic failure
 

Elemental analysis is used to evaluate and quantify wear metal elements, additive elements and contamination elements. Wear metals are analyzed to pinpoint problem areas through trend analysis. By analyzing the additive elements, the oil type can be verified, i.e., hydraulic oil, transmission fluid, or engine oil. Contamination elements are reviewed to determine lubricant serviceability and to pinpoint causes of problems indicated by other test results.
 
Following are the sources of the elements analyzed and their function in a component:
 

Terms

Detergent-Additive which keeps the engine clean at high operating temperature.
Dispersant-Additive which keeps debris in suspension in the oil, controls deposits at moderate temperature.
Anti-wear (AW)-Additive which provides a protective film.
Extreme Pressure (EP)-Additive which provides a protective film in high-pressure areas.
NOTE: While some elements indicated are additives they may at the same time be from elsewhere. For example Calcium the additive is NOT differentiated from Calcium taken in from road dust which may contain Calcium Chloride. One should always be aware of surprising increases in additive elements. They do NOT naturally increase.