Frequently Asked Questions >> Test Descriptions >> Oil Tests

Oil Tests

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Additives & Contaminants. An additive is a chemical substance added to a petroleum product to impart or improve certain properties. Common additives are: antifoam agent, anti-wear additive, corrosion inhibitor, demulsifier, detergent, dispersant, emulsifier, EP additive, oiliness agent, oxidation inhibitor, pour point depressant, rust inhibitor, tackiness agent, viscosity index improver. A contaminant is any foreign or unwanted substance that can have a negative effect on system operation, life or reliability.

Fuel Dilution. Measures the amount of fuel contamination in the oil, specifically the amount of raw, unburned fuel that ends up in the crankcase. Dilution lowers an oil's viscosity -- creating friction-related wear almost immediately -- and decreases unit load capacity. Reported as a percentage of volume.

Fuel Soot. Used to determine combustion efficiency. Soot can be caused by over-fueling, air restrictions, blow-by, excessive engine brake use and/or excessive exhaust back-pressure. Reported as a percentage of volume.

Glycol. Glycol is a test used to check an oil for contamination from a glycol product such as antifreeze. Levels should range from 40% to 60% to ensure proper freeze point protection. A high percentage of glycol can cause additive drop out and shorten coolant life.

Nitration. Nitration products are formed during the fuel combustion process in internal combustion engines. Most nitration products are formed when an excess of oxygen is present. These products are highly acidic, form deposits in combustion areas and rapidly accelerate oxidation. Nitration indicates excessive "blow-by" from cylinder walls and/or compression rings. It also indicates the presence of nitric acid, which speeds up oxidation. As oxidation/nitration increases, so will total acid number and viscosity, while total base number will begin to decrease.

Oxidation. Oxidation measures the breakdown of a lubricant due to age and operating conditions. Oxidation occurs when oxygen attacks petroleum fluids. The process is accelerated by heat, light, metal catalysts and the presence of water, acids, or solid contaminants. It leads to increased viscosity and deposit formation. It prevents additives from performing properly and therefore allows acid content and viscosity to increase.

Particle Count. A measure of all particles that have accumulated within a system, including those metallic and non-metallic, fibers, dirt, water, bacteria and any other kind of debris. It is most useful in determining fluid and system cleanliness in filtered systems such as hydraulics, turbines, compressors, auto/power shift transmissions, recirculation systems and filtered gear systems.

pH. pH is a measure of oil's alkalinity or acidity. It indicates the intensity of acid-forming or base-forming materials present.

Total Acid Number. Measures the amount of acid present in the oil. Technically, the quantity of base (expressed in milligrams of potassium hydroxide) required to neutralize all acidic constituents present in one gram of sample. Total Acid Number is the Numbers higher than that of new lubricant is an indication of oxidation or contamination of some kind.

Total Base Number. Measures the oil's alkaline reserve, or ability to neutralize acid. Technically, the quantity of acid (expressed in milligrams of potassium hydroxide) required to neutralize all basic constituents present in one gram of sample.

Viscosity. A measure of a lubricant's resistance to flow (fluid thickness) at temperature. Viscosity is considered the most important physical property of an oil. Depending on lube grade, viscosity is tested at 40° C. and/or 100° C. Reported in Centistokes.

Viscosity Index. Viscosity Index represents an oil's change in viscosity with respect to changes in temperature. The viscosity index of an oil is determined experimentally by testing its viscosity at 40° C. and 100° C.

Water. Measure the amount of water in the oil. Water can be measured by the crackle method (using a hot plate) or by the Karl Fisher method (titration). Water in oil decreases lubricity, prevents additives from performing properly and furthers oxidation.


Viscosity is a measure of an oil’s resistance to flow. It is the most important property of an oil.

It is critical for the oil to be in the right range for proper engine lubrication. If the viscosity is too low, the oil will cause excessive engine wear. If it is too high, the oil may not provide sufficient flow to critical engine components.

Viscosity is dependent on temperature. Generally, viscosity will decrease as temperature increases. The change in viscosity with temperature is referred to as the "viscosity index" (VI).

Factors Affecting Viscosity

In use, an oil’s viscosity will increase or decrease from new oil viscosity. Increases in viscosity are due to high temperature oxidation, soot accumulation, water or coolant contamination. A decrease in viscosity usually signals fuel dilution; however, in multigrade oils, it may be due to a shearing of the viscosity index improver additive used in these types of oils.

Engine conditions that change the oil’s SAE viscosity grade can be detrimental to engine life. Indications of this type require an immediate oil and filter change. In multigrade oils, a loss in viscosity due to shearing of the VI improver additive is a characteristic of the brand of oil used and can only be corrected by changing oil brands. Viscosity changes caused by engine contamination must be investigated and corrected.

Measurement of Viscosity

The most common technique for measuring viscosity is kinematic viscosity, as defined by ASTM D445. This test method measures the time it takes for a known volume of oil at a specific temperature to flow under gravity through a specially designed precision glass tube. Different types of tubes are used depending on whether the oil is new or used. Kinematic viscosity is measured at two temperatures -- 40°C. (approximately 100°F.) and 100°C. (approximately 210°F.) -- to simulate ambient temperatures and high engine coolant temperatures.

Both measurements may be used in new oils to calculate viscosity index. VI values greater than 100 indicate a multigrade oil, and values less than or equal to 100 are monograde oils.

While the viscosity measured at 100° C. is used to determine the oil grade, the viscosity at 40° C. provides a better indication of fuel dilution or water contamination. These contaminants generally boil off at higher temperatures, and they are often not detected by changes in viscosity. An increase of 40% or a decrease of 15% from new oil viscosity at 40° C. indicates that there may be a problem with the oil. In this case, we would recommend a confirming test for fuel dilution.

Typical Viscosity Values

Viscosity Grade 15W-40 30 40 50
Viscosity, Kinematic, cST        
@ 40° C. 95 - 115 100 - 120 130 - 150 200 - 230
@ 100° C. 12.5 - 16.3 9.3 - 12.5 12.5 - 16.3 16.3 - 21.9
HT/HS, cP 150° C. 3.7 minutes --- --- ---
Viscosity Index 130 95 95 95

Fuel Dilution

Contamination of engine oil by fuel may occur from incomplete combustion due to extended periods of idling and/or the use of a heavy fuel. Fuel dilution may also be caused by overfueling from a malfunctioning or improperly sized fuel injector. Poor fuel atomization and leaking are two likely injector malfunctions.

A small amount of engine oil fuel dilution occurs in normal engine operation. It becomes a potential problem when it causes a significant lowering of oil viscosity. As a general rule, you can generally tolerate up to 5% by volume, but you should investigate any reading over 2.5%, regardless of oil hours or miles. The engine and oil are less tolerant of some fuels than others. Some fuels (such as high-sulfur fuel, methanol, ethanol and biodiesel) may cause harm below 2.5% by volume. Total base number (TBN), wear metals and viscosity may provide additional information for a particular engine or application. 

Measurement of Fuel Dilution

The two test methods for fuel dilution are flash point reduction (ASTM D92) and gas chromatography (ASTM D3524). Gas chromatography is considered to be more sensitive and precise. Gas chromatography is a technique in which the fuel is separated from the oil by passing the heated mixture through a special column which permits faster passage of the more volatile fuel components. The relative quantities of fuel and oil are determined by a flame ionization detector that differentiates these components.

Water Content

Water contamination of engine oils in concentrations as little as 500 ppm can seriously affect the oil’s filterability and the function of the oil’s additive package. The detection of water in concentrations above 0.3% by volume (about 3,000 ppm warrants immediate corrective action and an oil change.

For screening purposes, water contamination may be detected effectively with a crackle test. This test is conducted either by dropping a small quantity of oil in a heated aluminum pan, or by immersing a hot electric soldering iron in a sample of the oil. A crackling noise indicates the presence of water.

An exact quantity of water may be determined by distillation (ASTM D 95) or by Karl Fischer titration (ASTM D 1744). The distillation test method measures total water only, while the titration method determines combined and free water. While the titration method can measure smaller quantities of water, it requires a new sample and may not give accurate results if the oil is badly contaminated by other materials.

In the distillation method, a measured quantity of the oil is dissolved in xylene and heated in a distillation flask. The water co-distills with the xylene and is collected in a graduated trap where the water settles to the bottom and the volume present is measured.

Glycol Content

Engine antifreeze is also a serious contaminant. As with water, small concentration of glycol can reduce filterability and functionality of the oil. In addition, large quantities of antifreeze can corrowde bearings and form a tar-like substance which can block oile galleries. Quantities above 1,000 PPM require corrective action and an immediate oil change, regardless of oil service hours or mileage. When large concentration of engine coolant are detected, the engine should be flushed out using an accepted procedure.

Glycol contamination is determined by spectrochemical analysis for sodium and boron, by gas chromatography (ASTM D 4291), or by a chemical test described in ASTM D 2982. This chemical test involves mixing a sample of the engine oil with several chemical reagents. The presence of glycol is confirmed with the presence of a purple color. The intensity of the color is directly related to the concentration of glycol present.

Soot Content

A characteristic of diesel fueled engines is that they produce soot in combustion. This combustion soot makes its way into the engine oil past the piston rings due to blow-by, engine timing, and operation. The quantity of soot in the engine oil is related to the engine operation, not the oil. However, the ability of the oil to function and protect the engine when soot is present is related to the oil’s performance. High concentration of soot can reduce wear protection and increase viscosity. The ability to handle some level of soot contamination is built into the oil formulation. The determination of soot concentration by itself is only of benefit in judging the operation of the engine. Restricted air intake, excessive idling, lugging, and worn cylinder kits are conditions which are generally related to high soot concentrations.

The condition of the oil with soot concentrations above 1.5% is evaluated in relation to wear metal content, viscosity, and total base number. An oil with normal condition indicators and with soot concentrations above 1.5% wold not necessitate an oil change. Conversely, condition indicators in the normal range even with low soot concentration would signal an oil change.

Total Base Number

Since most contaminants and engine combustion by-products are acidic, engine oil formulations are designed to neutralize these acids and, therefore, tend to be alkaline. As an oil ages with use, the alkalinity decreases, signaling a need to change the oil. Total Base Number (TBN) can only be replenished by an oil change. Operating with an oil with too low a TBN can cause increased wear and deposit formation.

An oil should be changed when the TBN drops to 1/3 of its new oil value. If high sulfur fuel (greater than 0.5% sulfur) is used, the oil should be changed at 1/2 of new oil TBN. TBN should never be below 2.0. TBN and viscosity are the most important parameters used to establish the condition of the oil. These must be monitored for optimizing oil drain intervals. 

TBN Test Methods

TBN is the amount of acid in milligrams required to acidify 10 milligrams of oil. It is a chemical test determined by titrating a known quantity of oil to a pH of 4.5, representing the most severe condition an oil would see in use. There are two TBN test methods:

  • ASTM D 2896 is intended to be used for new oils and measures weak and strong alkaline components.
  • ASTM D 4739 recommended for used oil analysis measures only the strong alkaline components.

ASTM D 2896 will give one to two TBN numbers higher than ASTM 4739 on the same oil. Although ASTM D 4739 is preferred for used oil analysis, either may be used. Be sure to compare TBN numbers from the same method when determining oil condition.

Spectrometric Analysis

Metals content in lubricating oil is determined by subjecting the oil sample to an excitation which creates spectral lines of the oil metals where their identity and concentration may be measured. Spectroscopic methods include atomic emission, inductively coupled plasma, atomic absorption, and X-ray fluorescence. All of these methods are capable of determining additive and wear metals.

Spectrometric analysis can be used to determine changes in oil formulation, relative engine component wear, and contamination. A common problem made in the interpretation of used oil analysis, particularly with spectrometric analysis, is whether one or more of the metals is above a limit. As discussed below, a trending approach is a better way to interpret results from a spectrometric analysis. Systematic errors from sampling and the analysis itself make the declaration of whether a single sample result is abnormal or critical somewhat suspect.

It must be recognized that only metals which have chemically reacted with the oil and particles below one micron in size are analyzed by this technique. Gross wear debris will not be detected. Generally, chemically generated wear debris tracks with particulate wear.

Infrared Spectroscopy

Infrared Spectroscopy (IR) is one of the most versatile test techniques available for both new and used oil analysis.

  • For new oils, IR can identify formulation changes, even when the metals’ fingerprints do not change.
  • For used oils, it can identify contamination from soot and water, depletion of antioxidant additives, and degradation from oxidation and nitration.

When used with other analysis techniques the IR becomes a very powerful analytical tool.

Fourier Transform Infrared (FTIR) is a relatively new variation to IR made possible by the availability of on-board instrument computers. FTIR has the advantage of speed, short analytical cycle time, small sample size requirements, and can adapt well to automation. The disadvantage of using IR more widely in used oil analysis is that a new oil sample is required and that interpretation of the data could be rather complex. The computerized instruments with their resident libraries of compounds are helping to minimize these concerns.

Flash Point

Flash point is normally determined for material safety and handling precaution requirements.