MRO Magazine

Understanding lubricant additives and their function

Formulated lubricants are various combinations of base oils and additives.

April 21, 2023 | By L. (Tex) Leugner

Photo: H_Ko / Adobe Stock

Photo: H_Ko / Adobe Stock

Additives reduce friction, dissipate heat, keep machine components clean, suspend carbon, sludge and varnish deposits and prevent metal damage due to wear, oxidation, and corrosion. Depending on the application, chemical additives will make up anywhere from 10 to 25 per cent of the lubricant. Equipment maintenance technicians and mechanics should have a full understanding of the type and purpose of additives in lubricants.

Certain metallic additives used in lubricants also reflect certain metals in equipment components and it is important to know the metallurgical makeup of the components in the plant equipment so as not to confuse a metal component with an additive element. For example, molybdenum is used in some lubricants as an anti-wear additive and can also be found as a surface coating on some piston rings. The additive types and their purposes are outlined below in the quiz questions.

Q | Can the equipment maintenance technicians provide examples of chemically active additives?
LOGIC: Chemically active additives are those that can interact chemically with the metal components to form a protective film by either adsorption or absorption; these include dispersants, detergents, anti-wear and extreme pressure agents, oxidation, rust, and corrosion inhibitors. Adsorption is the plating out of an anti-wear additive on a metal surface to provide a protective film. Absorption is the assimilation of an additive into the lubricant to improve a specific requirement such as that of adding friction modifiers to the lubricant itself.

Q | Does the maintenance technician know the chemically active additives that are listed on the oil analyses reports that are used to monitor equipment condition?
LOGIC: Common additives used in industrial lubricants are detergents such as sodium, magnesium, manganese, calcium and barium. However, be careful when interpreting oil analyses reports, because these elements sometimes confirm other conditions; for example, sodium is sometimes used as a coolant additive. Magnesium is used as a constituent in certain aluminum alloys. Boron is used as an anti-freeze additive, a chemically active extreme pressure additive, while some boron additives are being used to reduce zinc dialkyl dithiophosphates (ZDDP).


Q | Can the equipment maintenance technician describe the purposes of chemically inert additives?
LOGIC: Chemically inert additives enhance the physical properties of the base oil itself to improve the lubricants performance, primarily through absorption. These additives include emulsifiers, demulsifiers, pour-point depressants, foam inhibitors and viscosity improvers. For example, viscosity improvers are used in all multi-grade oils to minimize changes in viscosity when temperatures change.

Q | Does the maintenance technician know the purpose of chemically inert additives that may be listed on the oil analyses reports?
LOGIC: Silicon is used as an anti-foam additive, but it is also used as an additive in some anti-freeze coolants, so if this appears on an engine oil analysis report in a higher-than-normal trend, it might indicate a contamination problem.

Q | Does the maintenance group have a lubrication specialist who understands the correct interpretation of the terms used that represent both chemically active and inert additives?
LOGIC: For example, ZDDP are oxidation and corrosion inhibitors and anti-wear additives. This additive is represented on an oil analyses report as ZDDP. The anti-wear element titanium may be found in some oils reducing ZDDP levels. Phosphorus is widely used as both an anti-wear and extreme pressure additive and depending on the oil analyses laboratories reporting system, may simply be shown as P on the report.

Q | Does the maintenance group have a specialist who understands the relationship between various additives and their ability to prevent oxidative or nitration conditions in the lubricant and how these might affect the equipment?
LOGIC: Acid number (AN) is a measurement of the acidic derivatives that cause oxidation of any industrial oil and is indicative of its remaining useful life. The oxidation of oil begins as hydroperoxides, and carboxylic acids develop result, and the oil begins to darken in colour. The AN of industrial oil is usually reported on the oil analyses report as TAN and a guideline for its use is that when the AN double that of the new oil’s AN, it is indicative that the service life of the used lubricant is over.

Q | Does the maintenance group have an engine lubricant specialist who understands the relationship between detergents and dispersants and their neutralizing ability?
LOGIC: Base number (BN) is a measurement of the reserve alkalinity remaining in engine lubricants and is directly related to the detergent/dispersant additive package and its ability to counteract acids, sludge and varnish and is reported on the oil analyses report as TBN. When the base number has been reduced by one-half of that of new oil, the used lubricant should be considered no longer serviceable.

Q | Does the maintenance group understand that lubricant analyses must include the quality of the lubricant itself, not only equipment wear condition?
LOGIC: Monitoring AN and BN respectively are excellent indicators of the lubricants remaining life and should be included for all oil analyses when oil quality and service life are important, such as when considering an extension of oil drain intervals. Monitoring the increase of AN or the decrease of BN respectively are the standards to apply.

Q | Does the maintenance group understand that lubricants contain additional additives that serve to protect more than just the lubricant itself, or the condition of the equipment?
LOGIC: Rust inhibitors provide a barrier between metal surfaces and harmful elements such as air or moisture. Sulphur is used in some extreme pressure oils and metal working fluids. It can be corrosive to yellow metals and the additive thiadiazole is in use to protect yellow metals found in worm gear drives. Others are pouring point depressants and viscosity index improvers, both helpful in ensuring that lubricant viscosities are maintained, particularly in northern climates where temperatures vary dramatically.

Finally, certain oil seal materials are not compatible with some lubricants, and it is important that replacement oil seals are of the correct type. Common seal materials are fluoro-elastomers, nitrile-rubber, polyacrylates and silicones. Certain base oils and additive systems can cause seals to shrink or become brittle and it is important that industrial lubricants contain both the correct type and amount of seal swell agents, such as polyesters or phosphorus derivatives.

A final word of advice; become familiar with the lubricant data sheets for the lubricants used in your plant equipment. Depending upon the lubricant manufacturer, these documents frequently list the additives used and new oil specifications of base or acid number, viscosities and pour points. Know and understand the purpose of oil additives.
L. (Tex) Leugner, author of Practical Handbook of Machinery Lubrication, is a 15-year veteran of the Royal Canadian Electrical Mechanical Engineers, where he served as a technical specialist. He was the founder and operations manager of Maintenance Technology International Inc. for 30 years. Tex holds an STLE lubricant specialist certification and is a millwright and heavy-duty mechanic. He can be reached at


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