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Fundamentals of Engine Oi Additives
AMSOIL Motor Oil Additives

What Is Motor Oil? – Part 3

Motor Oil Additives

When Motor Oil is blended, many chemicals (additives) are added to the oil.

Additives impart new characteristics to the oil, or improve existing characteristics, enabling it to function in a desired manner when used to lubricate an engine. Additives improve the overall performance of the fluid.

Additives are used to enhance the beneficial properties of the base oil, helping it stand up to extreme operating environments, and make up for its deficiencies. Even the best base oil would not be able to protect as well against the effects of heat, shearing forces, chemical and water dilution, corrosion and wear particles. In short, additives make good base oils even better.

When using oil additives, more is not always better. As more additive is blended into the oil, sometimes there isn’t any more benefit gained, and at times the performance actually deteriorates. In other cases, the performance of the additive doesn’t improve, but the duration of service does improve.

In addition, increasing the percentage of a certain additive may improve one property of an oil while at the same time degrade another. When the specified concentrations of additives become unbalanced, overall oil quality can also be affected.

Some additives compete with each other for the same space on a metal surface. If a high concentration of an anti-wear agent is added to the oil, the corrosion inhibitor may become less effective. The result may be an increase in corrosion-related problems. (See: “Can I Mix Different Oils?”)

The various chemicals that comprise the additive system help modern Lubricants meet the increasing demands of today's high-tech engines. For passenger car motor oils, the base oils make up 70 to 80 percent of the final product; the other 20 to 30 percent is comprised of additive chemistry.

It’s important to note that most additives are also sacrificial. Once they are gone, they’re gone. A proper oil analysis report can determine the health of the additives remaining in the lubricant.

Friction Modifiers

Friction is the force that resists relative motion between two bodies in contact. If friction didn’t exist, nothing would ever stop moving. We need friction to function, but there are instances where you want to be able to reduce the amount of friction present.

One of oil's main functions is to prevent/reduce friction and wear. As the metal surfaces come in contact with one another, there is heat generated by means of friction and pressure. Friction modifiers are used for the purpose of reducing friction and noise, prevent wear and scoring, and improving fuel economy.

Friction Modifiers alter the frictional properties of a lubricant and can be used to give oil more 'slippery' characteristics.

Friction Modifiers, and mild anti-wear agents, are polar molecules added to lubricants for the purpose of minimizing light surface contacts (sliding and rolling) that may occur in a given machine design and increase the oil’s lubricity. These are also called boundary lubrication additives.

These molecules have a polar end (head) and an oil-soluble end (tail). Once placed into service, the polar end of the molecule finds a metal surface and attaches itself. If you could see the orientation of the molecules on the surface, it would appear something like the fibers of a carpet, with each molecule stacked vertically beside the others.

As long as the frictional contact is light, these molecules provide a cushioning effect when one of the coated surfaces connects with another coated surface. If the contact is heavy, then the molecules are brushed off, eliminating any potential benefit of the additive.

Anti-Wear Agents

When the machine designer anticipates more than light surface contact (from shock loading, for instance), then he would select a stronger type of friction modifier characterized as an anti-wear agent.

Anti-wear agents chemically react with the metal surfaces during operation at elevated temperatures to form a film barrier between moving parts that helps prevent metal-to-metal contact.

AW additives work specifically to protect metal surfaces during boundary lubrication conditions. They form a ductile, ash-like sacrificial film at moderate to high contact temperatures reducing wear. Under boundary conditions, the AW film shears instead of the surface material.

In reacting with the metal surface, these additive types form new compounds such as iron chlorides, iron phosphides and iron sulfides (dependent upon which compound is used). The metal salts produce a chemical (soap-like) film that acts as a barrier to reduce friction, wear and metal scoring, and eliminate the possibility of ‘welding’.

Zinc dialkyldithiophosphate (ZDDP) is a common anti-wear agent. This type of additive literally reacts with the metal surface when the reaction energy (temperature) is high enough. The reaction layer provides sacrificial surface protection.

Note: Anti-wear agents perform in a similar manner to Extreme-Pressure agents, but tend to operate under lower loads, temperatures, and pressures.

Extreme-Pressure Agents

Extreme-Pressure agents coat metal surfaces to help prevent close-contact components from scoring and seizing under extreme heat and pressure.

They are activated by high temperatures and high loads to react with the metal’s surface to form a sacrificial wear layer on components. As the loading and metallic contact increase, the strength of the additive and reaction process increases.

This leads to the use of sulfur-phosphorus based extreme pressure (EP) chemicals.

The EP additives form organo-metallic salts on the loaded surfaces that serve as sacrificial films to protect against aggressive surface damage. Other solid materials such as graphite and molybdenum disulfide are also used as EP agents.

Extreme-Pressure Agents are used in gear oils, other power-transmitting fluids, load bearing greases, and metalworking fluids.  They are usually supplemented with anti-wear additives to make them effective across a wide range of conditions.

Many effective extreme pressure agents and anti-wear additives (chlorine for instance) are corrosive to metals, so they’re typically formulated to balance protection with corrosion protection.

There are two main types of EP additives, those that are temperature-dependent, and those that are not.

The most common temperature-dependent types include boron, chlorine, phosphorus and sulfur. They are activated by reacting with the metal surface when the temperatures are elevated due to the extreme pressure. The chemical reaction between the additive and metal surface is driven by the heat produced from friction.


Combustion causes carbon build-up and deposit formation on the pistons, rings, valves and cylinder walls negatively affecting engine temperature and performance, oil circulation, and fuel efficiency. Some combustion by-products slip past the piston rings and into the motor oil, which can clog the engine’s oil channels.

Detergents clean engine surfaces where deposits may be detrimental. They help suspend and disperse contaminants in the oil to keep them from accumulating on component surfaces which can lead to the formation of sludge and other deposits.

Detergents also protect surfaces against carbon and varnish buildup. Additionally, they also work to neutralize acids and the corrosive effects of combustion and oxidation bi-products.

Detergents include primarily metallic salts such as calcium, magnesium and sodium.

They are most efficient at controlling high-temperature deposits.


While detergents help minimize the amount of combustion by-products, dispersants hold those contaminants suspended and dispersed in the oil keeping engine surfaces free of sludge and deposits. The Larger suspended particles are removed by the oil filter.

Dispersants minimize the tendency of contaminants to agglomerate into large lumps which settle out as sludge or varnish. Although both detergents and dispersants act as cleanliness agents, they differ chemically.

Dispersants are non-metallic materials and are more effective than metallic detergents at controlling deposits under intermittent or low-temperature operations.

Additive polarity is defined as the natural directional attraction of additive molecules to other polar materials in contact with oil. In simple terms, it is anything that water dissolves or dissolves into water.

Dispersants are attracted to contaminants, such as dirt, soot, and water. They will cling to the particle and envelop it, preventing it from agglomerating and forming deposits on surfaces. They will then settle to the bottom of the oil pan or be filtered out by the oil filter, depleting your additive package.

Dispersants also fight the build-up of corrosive acids and are most efficient at controlling low-temperature deposits.


Oxidation is a degradation process that occurs when atmospheric oxygen reacts with organic molecules.

Like any chemical, oils can undergo chemical reactions that change the structure of molecules. For the most part, such reactions are undesirable because they lead to a change in the properties of the lubricant such that it is less effective at reducing friction and preventing corrosion.

Oil oxidation produces acidic gases in the crankcase which, when combined with water, leads to corrosion and rust. For every 10°C increase in operating temperature, the oxidation rate of an unprotected lubricant almost doubles.

Excessive engine heat causes the oil to chemically break down which results in permanent thickening and degradation of the oil and the formation of sludge.

Antioxidants work to limit the impact of oxidation by reducing the tendency for oil to react with oxygen. They also retard oil oxidation reducing sludge buildup.

Rust & Corrosion Inhibitors

Rust and corrosion are caused by the attack of acidic bi-products on unprotected metal surfaces in the presence of water and oxygen. Unfortunately, these elements are present in abundance in internal combustion engines.

Rust & Corrosion Inhibitors form a protective barrier over component surfaces to seal out water and oxygen and other “oxidizers” that can cause chemical reactions to occur that lead to damage to the surface of a material.

While most rust and corrosion inhibitors work by forming a physical barrier on the component surface, some rust inhibitors function by neutralizing acids.

Antifoamant Additives / Foam Inhibitors

When air bubbles are whipped into motor oil by the action of the many rapidly moving parts, it results in a mass of oily froth that has very little ability to lubricate or cool the engine.

Foaming can lead to inadequate lubrication, cavitation and mechanical failure.

Antifoamant additives weaken the air bubbles, reducing their surface tension, causing them to collapse almost immediately upon forming and allowing the oil to continue protecting the engine.

Seal Swell Agents

Motor oil must be compatible with the various seal materials used in engines. It must not cause seals to shrink, crack, degrade or dissolve.

Seal Swell agents cause seals to expand or swell slightly to ensure continued proper sealing in order to prevent leaks.

Pour Point Depressants (PPDs)

Pour Point is an indicator of the ability of oil to flow in sub-zero operating temperatures. It is the lowest temperature at which the fluid will flow. A pour point depressant lowers that temperature.

Modern refining techniques remove most of the wax (paraffin) from petroleum oil, but some wax molecules remain. These wax molecules are soluble at ambient temperatures above freezing, but crystallize and agglomerate at lower temperatures effectively increasing the oil’s viscosity, causing oil pumpability and circulation problems.

Pour Point Depressants are designed to inhibit the formation of these wax crystals, minimizing viscosity increase, and prevent them from agglomerating or fusing together at freezing temperatures.

This in turn lowers the temperature at which the oil will pour and flow. The result is easier starting, less engine wear and longer engine life.

Pour Point Depressants are found in most motor oils designed for cold weather use.

Viscosity Index Improvers (VII)

The viscosity (thickness) of a liquid is a measure of its internal resistance to flow – the thicker the oil, the higher its viscosity.

The viscosity of oil is affected by temperature changes during use. At hotter temperatures, the oil becomes thinner (viscosity decreases) and provides less engine protection. At colder temperatures, the oil thickens (viscosity increases) and becomes more difficult to pump around the engine, resulting in less protection at start-up.

The Viscosity Index (VI) is a measure of how much the oil's viscosity changes with changes in temperature. The higher the viscosity index, the less the oil’s viscosity changes with changes in temperature. In other words, oils with higher viscosity indices thin less at higher temperatures and don’t thicken as much at lower temperatures.

The Viscosity Index is simply reported as a numerical value that has no units. The measurements are taken at 40°C and 100°C.

Viscosity Index Improvers (VII) are special long-chain polymers – sometimes referred to as Viscosity Modifiers (VM) – that are added to motor oil to prevent it from thinning as much as it normally would as it gets hotter. In other words, VII slow down the rate at which oil thins out as the temperature rises.

The VII polymers expand and contract as temperatures vary. High temperatures cause the molecules to expand and reduce oil thinning; low temperatures cause them to contract and have little impact on oil viscosity (see picture below).

Oil Viscosity Index Improvers

Viscosity Index Improvers come in different shapes, sizes and quality levels (i.e. some polymer chemistries are better thickeners than others). Generally speaking, larger molecules are better thickeners than smaller ones; however, they are also more easily broken, which impacts the shear stability of the oil.

An example of a VII is an Olefin Co-polymer (OCP) which is a co-polymer of ethylene and propylene.

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