Glossary of Terms

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  Absolute (or Dynamic) Viscosity

A measure of resistance to flow of a fluid. The ratio of applied shear stress to rate of shear. The units of Dynamic viscosity are milli Pascal-seconds in the ISO system and centipoise in metric.

Acid Number

Same definition now listed for 'Neutralization Number". The weight, in milligrams of potassium hydroxide needed to neutralize acids in one gram of oil by ASTM method D 664 or D 974. The acid number of an oil is a measure of the presence of reactive additives or oil oxidation.

Acid Treating

A refining process in which unfinished petroleum products, such as gasoline, kerosene, and lubricating oil stocks are contacted with sulfuric acid to improve their color, odor, and other properties.


The amount of free acid in any substance.


An agent used for imparting new, or for improving existing characteristics of a lubricating oil or grease.

Additive Level

The total percentage of all additives in an oil.


Abbreviation for American Gear Manufacturers’ Association.

Air Entrainment

The incorporation of air in the form of bubbles as a dispersed phase in the bulk liquid. Air may be entrained in a liquid through mechanical means and/or by release of dissolved air due to a sudden change in environment. The presence of entrained air is usually readily apparent from the appearance of the liquid (ex: bubbly, opaque, etc.) while dissolved air can only be determined by analysis.

Aniline Point

The minimum temperature for complete miscibility of equal volumes of aniline and the sample under test. ASTM Method D 611 describes procedures for determining aniline point and mixedaniline point of petroleum products and hydrocarbon solvents. Aniline point is often specified for spray oils, cleaning solvents, and thinners, where effectiveness depends upon aromatic content. In conjunction with API gravity, the aniline point may be used to calculate the net heat of combustion of aviation fuels.

Anti-foam Agent

An additive used for controlling foam.

Antifriction Bearing

A type of bearing employing rollers or balls. (rolling bearings)


Resistance to detonation or pinging in spark-ignition engines


A chemical added to gasoline, lubricating oil, etc., to inhibit oxidation.


American Petroleum Institute

API Engine Service Classification System  Classification and designations for lubricating oils for automotive engines developed by API in conjunction with SAE and ASTM.

API Gravity

An arbitrary scale expressing the gravity or density of liquid petroleum products. The measuring scale is calibrated in terms of degrees API. It may be calculated in terms of the following formula:

Deg API =               141.5                      

                       sp gr. 60°F/60°F

Ash Content

The percent by weight of residue left after combustion of a sample of a fuel oil or other petroleum oil; it is usually determined in the United States by ASTM Method D 482.


American Society of Lubrication Engineers. Name changed to STLE.


American Society for Testing and Materials. Develops test methods and product specifications.

ASTM Colorimeter

Apparatus widely used for determining the color of lubricating oil; it is described in ASTM Method D 1500. The color so determined is known as ASTM color.

ASTM Melting Point

The temperature at which D 87 wax first shows a minimum rate of temperature change; also known as the English melting point.

ASTM Viscosity Classification

A method of specifying viscosity levels for industrial lubricants; does not connote quality, D 2422

Boundary Lubrication

A state of lubrication existing when conditions of bearings, design, feed, load, and method of application of the lubricant do not permit the formation of a separating lubricant film by hydrodynamic action. Under these conditions, absorption of the lubricant or of some of the active components of the lubricant upon the bearing surface, or the formation of low shear-strength chemical compounds by the reaction of the components of the lubricant with the bearing surfaces, reduces the metallic contact and determines the character of the frictional resistance.

Bright Stock

Refined, high-viscosity lubricating oils usually made from residual stocks by suitable treatment, such as a combination of acid treatment or solvent extraction with dewaxing or clay finishing.

British Thermal Unit (BTU)

The quantity of heat required to raise, by 1°F, the temperature of one pound of water at its maximum density (39.2° F).

Centipoise (cP)

The metric units of Dynamic viscosity, numerically identical to the ISO units of milli Pascal seconds. 1 cP = 1 mPa•s

Centistoke (cSt)

The metric units of Kinematic viscosity, numerically identical to the ISO units of millimeter squared per second. 1 cSt = 1 mm2/s

Cetane Number (Calculated)

The cetane number of distillate fuels as estimated from the API gravity and mid-boiling point by using a formula given in Appendix II of ASTM Method D 975. This estimate is used if a standard test engine is not available, or if the sample is too small for an engine test.


Code of Federal Regulations.


1. The phenomenon observed among gear lubricants and greases when they thicken, due to cold weather or other causes, to such an extent that a groove is formed through which the part to be lubricated moves without actually coming in full contact with the lubricant.

2. A term used in percolation filtration; may be defined as a preponderance of flow through certain portions of the clay bed.

Cleveland Open-Cup (COC) Tester

Apparatus used for the determination of flash and fire points of all petroleum products flashing above 175°F, with the exception of fuel oils.

Cloud Point

With respect to a petroleum oil, the temperature at which paraffin wax or other solid substances begin to crystallize or separate from the solution, imparting a cloudy appearance to the oil when the oil is chilled under prescribed conditions. These conditions are described in ASTM Method D 97.


A factor in the identification, rather than in the quality rating of a petroleum product except where staining or appearance are considerations.

Copper Strip Corrosion

A test method (D130) which measures the tendency of a lubricant or fuel to corrode a strip of copper metal under standard test conditions.


The gradual eating away of metallic surfaces as the result of oxidation or other chemical action. It is caused by acids or other corrosive agents.


The mass of a unit of volume of a substance.

Detergent Oil

A lubricating oil possessing special sludge-dispersing properties for use in internal combustion engines. These properties are usually conferred on the oil by the incorporation of special additives. Detergent oils hold sludge particles in suspension and thus promote engine cleanliness.

Dielectric Strength

A measure of the adequacy of insulating materials for the electrical stresses they are intended to resist. Testing of electrical insulating oils of petroleum origin for use in cable, transformer, oil circuit breakers, and similar apparatus is usually done in the United States by ASTM Method D 877.

Diester Oil

A synthetic lubricating fluid made from esters; also called ester oil.


A dispersing agent, compatible with a carrier fluid, which holds a very finely divided third substance in a dispersed state in the carrier fluid.

Dropping Point

In general, the dropping point is the temperature at which the grease passes from a semisolid to a liquid state. This change in state is typical of greases containing, thickeners, and conventional type soaps. Greases containing, as thickeners, materials other than conventional soaps may, without change in state, separate oil. The method is useful in identifying the grease as to its type and for establishing and maintaining benchmarks for quality control.

Dry Film Lubricant

Solid material left between two moving surfaces to prevent metal-to-metal contact, thus reducing wear. Such materials are useful in the region of boundary lubrication, and for lubrication under conditions of extreme high or low temperatures where normal lubricants are inadequate. Some examples are graphite, molybdenum disulfide, boron nitride, and certain plastics such as tetrafluorthylene resins 


A substance used to promote or aid the emulsification of two liquids and to enhance the stability of the emulsion.

EP Agent

An extreme pressure additive introduced into a lubricant to impart load-carrying or anti-weld qualities.

EP Lubricant

Extreme pressure lubricant: any of the lubricating oils or greases which contain a substance or substances specifically introduced to prevent metal-to-metal contact in the operation of highly loaded gears. In some cases, this is accomplished by the substance reacting with the metal to form a protective film.


An animal or vegetable oil which will combine with an alkali to break down fat to form a soap.


Food & Drug Administration

Filler (Lubricants)

Any substance, such as talc, mica, or various powders, which may be added to a grease to make it heavier in weight or consistency, but which serves no useful function in making the grease a better lubricant.

Film Strength

The property of an oil which enables it to maintain an unbroken film on lubricated surfaces under operating conditions, where otherwise there would be scuffing or scoring of the surfaces.

Fire Point

The lowest temperature at which, under specified conditions in standardized apparatus, a petroleum product forms an air vapor mixture which burns continuously when ignited by a small flame.

Flash Point

The lowest temperature at which vapors arising from the oil will ignite momentarily (i.e., flash) on application of a flame.

Floc Point

The temperature at which wax or solids begin to form.


A froth produced by whipping air into a lubricant.

Four-Ball Tester

Equipment used to evaluate a lubricant's anti-wear qualities, frictional characteristics, or load carrying capabilities. It derives its name from the four 1/2 inch steel balls used as test specimens. Three of the balls are held together in a cup filled with lubricant while the fourth ball is rotated against them.

Fretting Corrosion

A process of mechanical attrition combined with chemical reaction taking place at the common boundary of loaded contact surfaces having small oscillatory relative motion.

FZG Test

A German gear test for evaluating anti-wear and EP properties.


General Agreement on Tariffs and Trade.


A lubricant composed of a lubricating fluid, thickened with soap or other material to a solid or semi-solid consistency.


A crystalline form of carbon either natural or synthetic in origin.

Heat Transfer Oil

A medium used for the transfer of heat at temperature levels above that of steam.

Herschel Demulsibility Number

A number which indicates the ability of anoil to separate from water under conditions specified by the Herschel Demulsibility Test.


The chemical addition of hydrogen to a material. In nondestructive hydrogenation, hydrogen is added to a molecule only if, and where, unsaturation with respect to hydrogen exists. In destructive hydrogenation, the operation is carried out under conditions which result in rupture of some of the hydrocarbon chains (cracking); hydrogen is added where the chain breaks have occurred.

Hypoid Gears

Gears in which the pinion axis intersects the plane of the ring gear at a point below the ring gear axle and above the outer edge of the ring gear, or above the ring gear axle and below the outer edge of the ring gear.


A substance, the presence of which, in small amounts, in a petroleum product prevents or retards undesirable chemical changes from taking place in the product, or in the condition of the equipment in which the product is used. In general, the essential function of inhibitors is to prevent or retard oxidation or corrosion.

Initial Boiling Point

According to ASTM Method D 86, the recorded temperature when the first drop of liquid falls from the end of the condenser.

Insulating Oil

An oil used in circuit breakers, switches, transformers, and other electrical apparatus for insulating, cooling, or both. In general, such oils are well-refined petroleum distillates of low volatility, with high resistance to oxidation and sludging.


International Standards Organization.

Kinematic Viscosity

A measure of resistance to flow of a fluid under gravity. The ratio of the absolute viscosity to the density at the temperature of the viscosity measurement. The kinematic viscosity of lubricating oils is commonly measured using ASTM Method D 445. The units are millimeters squared per second in the ISO system, centistokes in metric and Saybolt Universal Seconds in (obsolescent) U.S. practice.


A term originally used to denote that which is "fit" and "proper". Most often, it is used to describe foods that are permitted to be eaten by Jewish dietary laws.


A status of Kosher food which is compatible with all other types of Kosher food, whether dairy or meat, when it is prepared on neutral equipment.

Lard Oil

An animal oil prepared from the fat of swine. These oils are compounded with mineral oils to yield lubricants of special wetting  properties, especially cutting oils.

Lead Naphthenate

A lead soap of naphthenate acid, which is soluble in mineral lubricants and imparts to them high film strength.

Liquefied Natural Gas (LNG)

Similar to LPG but consisting of lighter hydrocarbons, such as methane and ethane.

Liquefied Petroleum Gas (LPG)

Light hydrocarbon material, gaseous at atmospheric temperature and pressure, held in the liquid state by pressure to facilitate storage, transport, and handling. Commercial liquefied gas consists essentially of propane, butane, or mixtures thereof.


The primary standard of capacity in the metric system, equal to the volume of one kilogram of pure water at maximum density, at approximately 4°C, and under normal atmospheric pressure.

MIL Spec

Military specification; a guide in determining the quality requirements of products used by the military services, published by the United States Department of Defense.

Multigrade Oil

One of the multi-viscosity number oils in which one oil combines several SAE viscosity number grades.

Multi-purpose Grease

A lubricating grease suitable to meet the individual requirements for chassis lubricant, bearing lubricant, joint lubricant, water-pump lubricant, and cup grease.


One of a group of cyclic hydrocarbons, also termed cycloparaffins or cycloalkanes. Polycyclic members are also found in the higher boiling fractions.

Neutralization Number

An obsolete term for acid number. The weight, in milligrams, of potassium hydroxide needed to neutralize the acid in 1 g of oil.

Neutral Oil

Light overhead cuts of lubricant stocks. Neutral oils are the basis for most commonly used automotive lubricants.


National Lubricating Grease Institute. Primary technical society concerned with grease products.

NLGI Number

One of a series of numbers classifying the consistency range of lubricating greases, based on the ASTM cone penetration number. The National Lubricating Grease Institute grades are in order of increasing consistency (hardness). See Appendix D.


  1. That characteristic of a liquid which is responsible for the degree of friction between two surfaces which cannot be accounted for on the basis of viscosity alone.
  2.  The ability of a lubricating oil to orient itself on bearing surfaces so as to form new surfaces with a low coefficient of static friction. 3. That characteristic which an oil must possess to a sufficient degree to enable it to overcome every frictional stress to which it is subjected.

Pale Oil

A petroleum lubricating or process oil refined until its color, by transmitted light, is straw to pale yellow.


Consistency, expressed as the distance in millimeters that a standard needle or cone penetrates vertically into a sample of the material under known conditions of loading, time, and temperature.

Pour Depressant

A lubricating oil additive which lowers the pour point of an oil containing wax by reducing the tendency of the wax to form a solid mass in the oil. Also called pour-point depressor, pour depressant.

Pour Point

The lowest temperature at which oil will pour or flow when it is chilled without disturbance under definite conditions. (ASTM Method D 97).

Pour Stability

The ability of a pour-depressant-treated oil to maintain its original ASTM pour point when subjected to storage at low temperatures approximating winter conditions.

Process Oil

An oil not used for lubrication but as a component of another material, or as a carrier of other products.


Rust and oxidation inhibited.


Society of Automotive Engineers.

Saybolt Universal Viscosity

The time, in seconds, for 60 mi of fluid to flow through a capillary tube in a Saybolt Universal viscometer at a given temperature using ASTM Method D 88.


Extreme pressure agents (sulfur, chlorine) employed in certain types of automotive gear lubricants.

Specific Gravity

The ratio of the weight (in air) of a given volume of a material to the weight (in air) of an equal volume of water at stated temperature.


Society of Tribologists and Lubrication Engineers (formerly ASLE). Primary technical society for industrial lubricants.


Saybolt Universal Seconds...the obsolescent U.S. units for kinematic viscosity, still commonly reported for some lubricants. SUS units are the time in seconds for 60 ml of fluid to flow through a capillary in a Saybolt viscosimeter. For many lubricants, the SUS viscosity is approximately five times the kinematic viscosity in centistokes 

Sulfated Ash

Defined in ASTM Method D 874 as the ash which remains after a sample of new additive-containing lubricating oil has been carbonized, and the residue subsequently heated with sulfuric acid to constant weight.

Sulfurized Oil

A product formed from mineral oil combined with sulfur or certain sulfur compounds. It has far greater film strength and load-carrying ability than straight mineral oil and is used as cutting oil.

Synthetic Oils

Oils produced by synthesis rather than by extraction or refinement.

TAN (Total Acid Number)

The quality of base, expressed in terms of the equivalent number of milligrams of potassium hydroxide, that is required to titrat the strong base constituents present in 1 g of sample.(SATM Method D 664 or D 974)

Timken EP Test

The Timken Extreme Pressure Test is one of many laboratory machines used in determining the load carrying capacities of oils and greases. In this test, a Timken bearing cup is rotated against a steel block. The highest load under which a lubricant prevents scoring of the steel block by the rotating cup is the reported value.


United States Department of Agriculture.

USDA Approvals:

H-1 applications where incidental food contact may occur.

H-2 applications where there is no possibility of food contact.

3-H applications as a release agent (direct food contact).

VI (Viscosity Index)

Viscosity index...an arbitrary scale used to show the rate of change of viscosity with temperature, defined by ASTM Method D 2270. Paraffinic mineral oils have a VI near 100; highly naphthenic oils have a VI near 0.


The measure of internal friction or resistance to flow of a liquid.

Worked Penetration

The penetration of a sample of lubricating grease immediately after it has been brought to 77°F and then subjected to 60 stokes in a standard grease worker. This procedure and the standard grease worker are described in ASTM Method D 217.