Glossary Of Oil Terms


Additive– an oil additive is a chemical compound that imparts new properties or enhances the inherent characteristic of the lubricant. For example, ZDDP is an additive that imparts anti-wear, oxidation control and corrosion resistance to a lubricant.

Air entrainment – the presence of air bubbles in oil as a result of agitation. For example, crankshaft windage or Vacuum pumps. Anti-foam additives help to speed the release of entrained air.

Anti-Foam additive – additives designed to reduce the foaming tendency of an oil or fluid by improving the air-release properties of the oil.

Anti-wear additive – additives designed to prevent galling and scuffing during boundary lubrication conditions. These additives function by forming a sacrificial coating on the metal surface, which prevents metal-to-metal contact. These additives, like ZDDP, are activated by heat and load.

Asperities – microscopic projections on metal surfaces, invisible to the naked eye, that create peaks and valleys. When two surfaces are in sliding contact, these imperfections cause interference that results in friction. Without proper lubrication, wear, scoring or welding will occur.

Babbitt – an alloy of copper, tin and antimony used for plain bearings.

Bernoulli’s Theorem – states that a rise in fluid velocity results in a drop in static pressure. It partially explains why lower viscosity oils produce lower oil pressure.

Boundary lubrication – lubrication between two rubbing surfaces in the absence of a full fluid lubricating film. Boundary lubrication is accomplished by the use of additives. Flat tappet valve trains and aggressive roller cam valve trains operate in boundary lubrication, which is why these engines require motor oils with increased ZDDP – a boundary lubrication additive.

Bulk Modulus – the resistance to compressibility of an oil or fluid. Critical in hydraulic systems like shock absorbers and power steering systems.

Channeling – a term describing the desirable tendency of a grease to form a channel in a bearing where the “shoulders” of the channel serve as grease reservoirs and seals.

Chelation – the reaction of a metal with another substance to remove the metal ion from the solution. Useful in treating minerals like calcium and magnesium that are common in municipal and well water. Untreated, these minerals can cause corrosion. Chelation binds these minerals and prevents corrosion.

Conventional oil– also known as petroleum or mineral oils are derived by refining crude oil

Corrosion Inhibitors – additives that “passivate” metal surfaces to protect the surface from corrosion reactions. Corrosion inhibitors can also function by neutralizing corrosive compounds. Corrosion inhibitors are very important additives for engines that operate in high moisture environments or see extended periods of storage.

Detergent – a metallic additive used in motor oils to clean deposits, prevent deposits and neutralize acids.

Dispersant – a non-metallic additive used in conjunction with detergent additives to prevent sludge and varnish in an engine.

Distillation – the first step in separating crude oil into its various components. Distillation uses a fractioning tower to separate the various hydrocarbons based on their boiling points. This is the first step in making the various hydrocarbon based products we are all familiar with – gasoline, diesel, motor oils, etc…

Ethanol– also known as ethyl alcohol, ethanol is an alternative fuel and an oxygenate for pump fuel. Modern pump fuel typically contains up to 10% Ethanol. In some cases, the Ethanol content in fuel can range from 15% Ethanol (E15)all the way up to E85 (85% Ethanol). Ethanol is both Hygroscopic and corrosive to Aluminum, Zinc and Steel, which can cause extensive damage to fuel system components like Carburetors, Fuel Pumps and Fuel Cells.

EP (Extreme Pressure) additives – these additives function similarly to Anti-Wear, except they activate at higher temperatures and greater loads. EP additives are commonly found in hypoid gear oils and EP greases. Compounds of sulfur, phosphorus and chlorine are common EP additives.

Elastohydrodynamic Lubrication (EHL) – a thin film form of lubrication that occurs under very high pressures where the contacting surfaces deform to create a “contact patch” that traps a small amount of lubricant that will separate the two surfaces. Think of this in terms of a slick tire deforming to create a contact patch with the track, and then running through water. The contact patch traps the water which causes the tire (and by association, the vehicle) to hydroplane. This form of lubrication is common in highly loaded roller bearings.

EPA – Environmental Protection Agency. EPA regulations have changed the chemical makeup of gasoline, diesel and motor oils in the last 10 years.

Galling/Scuffing – condition where surfaces come into contact and excessive friction results in localized welding of surface asperities with subsequent metal transfer and further surface roughening.

Grease – a lubricant composed of an oil thickened with a soap or other thickener.

Gum – a sticky, rubbery deposit, black or brown in color resulting from the oxidation of unstable components in gasoline that deposit during use or in storage.

HTHS (High Temperature High Shear)– HTHS is a measure of viscosity under conditions of high temperature and high shear. HTHS predicts bearing oil film thickness. High HTHS oils provide greater wear protection, but typically rob horsepower and fuel economy. Oils with lower HTHS are thinner and can improve fuel economy by reducing drag throughout the engine, although too low of an HTHS may come at the expense of reduced wear protection.

Hydrodynamic lubrication – condition where the lubricant is able to fully separate the two surfaces with a full lubricant film based upon the relative speed of the surfaces, the load being carried and the viscosity of the lubricant.

Hygroscopic – hygroscopic substances easily absorb moisture from the environment, meaning that they can collect water over time. High water content in either fuel or oil can result in corrosion, gummy deposits and increased oxidation. Hard starting and poor performance are typical symptoms of excess water.

Hydrometer – an instrument used to measure the specific gravity of an oil or fuel. Commonly referred to as the “water test”.

Lacquer – a deposit resulting from the oxidation and polymerization of the lubricant and/or fuel due to exposure to extreme temperatures.

Lubrication – reduction of friction and wear between two load bearing surfaces by the application of a lubricant. There are 4 modes of lubrication – Boundary lubrication (thin, chemical film lubrication), Mixed film (partial oil films), EHL (pressure induced thin film lubrication), Hydrodynamic lubrication (full fluid film lubrication).

Methanol– Methyl Alcohol is also an alternative fuel and a common racing fuel. Because it has a high octane rating, methanol is best suited for high compression internal combustion engines. Methanol is also hygroscopic and corrosive. Due to the high oxygen content of Methanol, the air fuel ratio must be adjusted to prevent a “lean” mixture, as a result, the amount of Methanol used relative to gasoline is nearly double.

Mixed film lubrication – condition where only partial oil films separate the two surfaces.

mPAO– a next-generation synthetic base oil. mPAO base oils feature a higher viscosity index, which allows for the formulation of multi-grade motor oils that are less dependent on traditional viscosity index improver additives.

Multi grade motor oil – an engine oil that meets both “winter” and “summer” SAE viscosity requirements. For example an SAE 10W-30 meets both the 10W “winter” performance requirements as well as the 30 grade “summer” requirements. Typically formulated with Viscosity Index Improver additives.

Octane: a reference standard that indicates the resistance to detonation/pre-ignition in gasoline engines. A higher Octane rating indicates greater resistance to detonation.

Oxidation – chemical process in which oxygen reacts with oil and or fuel that results in acids and polymers that lead to corrosion and deposits.

PPM – parts per million

Profilometer – a device that measures and profiles the smoothness and roughness of a surface.

Rheology – the study of how viscosity changes due to increases in shear over temperature and time.

Shear Stability – a measure in the change in viscosity after a oil/fluid is subjected to shear over a range of temperature and time. Lubricants with poor shear stability lose viscosity in service due to high shear and high temperature conditions. Lubricants with excellent shear stability maintain their viscosity in service despite high temperatures and high shear.

Synthetic oil– these “custom built” oils are a result of controlled chemical synthesis. Synthetic oils feature uniform and tailored chemical structures that allow them to provide benefits beyond conventional oils.

Tribology – the science and technology of interacting surfaces in relative motion. Tribology combines the various disciplines of chemistry, metallurgy, machine design, and lubrication engineering to solve issues related to friction and wear.

Viscosity– Simply put, viscosity is an oil’s resistance to flow. Commonly measured in Centistokes, the greater the resistance to flow, the higher the viscosity and the greatest the Centistoke value. Lower viscosity oils flow better than high viscosity oils, which demonstrates less resistance to flow (lower Centistoke value). It is important to remember that viscosity changes with temperature and shear (see viscosity index and shear stability).

Viscosity Index Improvers – polymer additives used in motor oil to increase the viscosity index of the motor oil.

Viscosity Index – a measure of the tendency of an oil to thin as temperatures increase. Motor oils are similar to maple syrup in that they both get thicker as they get colder and get thinner as they get hotter. Maple syrup has a low viscosity index – it thickens rapidly when it cools and thins rapidly when it gets hot. The slower the rate of viscosity change with increasing or decreasing temperature, the higher the viscosity index. Higher viscosity index oils tend to reduce wear in engines.

Volatility– the measure of an oil’s tendency to vaporize at high temperature. Studies indicate high volatility oils lead to increased oil consumption and contribute to intake valve and combustion chamber deposits.

Zinc (ZDDP)- ZDDP – Zinc DialkylDithioPhosphate is a family of anti-wear chemical compounds. They provide multiple benefits. ZDDP additives not only provide anti-wear protection for rubbing surfaces, they also help prevent oxidation and corrosion.