Detergent additives:Dating back to the early 1930s, are used to clean and neutralize oil impurities which would normally cause deposits (oil sludge) on vital engine parts.
Friction modifiers: Like molybdenum disulfide,are used for increasing fuel economy by reducing friction between moving parts. Friction modifiers alter the lubricity of the base oil. Whale oil was used historically.
Viscosity modifiers: Make oil’s viscosity higher at elevated temperatures, improving its viscosity index (VI). This combats the tendency of the oil to become thin at high temperature. The advantage of using less viscous oil with a VI improver is that it will have improved low temperature fluidity as well as being viscous enough to lubricate at operating temperature. Most multi-grade oils have viscosity modifiers. Some synthetic oils are engineered to meet multi-grade specifications without them.
Corrosion or rust inhibiting additives:Retard the oxidation of metal inside an engine.
Antioxidant additives:Retard the decomposition of the stock oil.
Antiwear additives or wear inhibiting additives: cause a film to surround metal parts, helping to keep them separated. Zinc dialkyldithiophosphate (ZDDP) is a popular antiwear additive and has showed lower wear number in flat tappet style engines (like the 4.0 from jeep), the use of which has been restricted due to potential damage to catalytic converters forced upon automakers by government regulation.
Pour point depressants: Improve the oil’s ability to flow at lower temperatures.
Anti-foam agents: Inhibit the production of air bubbles and foam in the oil which can cause a loss of lubrication, pitting, and corrosion where entrained air contacts metal surfaces.
Seal conditioners: Cause gaskets and seals to swell so that the oil cannot leak by.
Metal deactivators: Create a film on metal surfaces to prevent the metal from causing the oil to be oxidized.
Extreme pressure agents:Bond to metal surfaces, keeping them from touching even at high pressure.
Dispersants: Keep contaminants (e.g. soot) suspended in the oil to prevent them from coagulating.
Wax crystal modifiers:Are dewaxing aids that improve the ability of oil filters to separate wax from oil. This type of additive has applications in the refining and transport of oil, but not for lubricant formulation.
Wear metals from friction: Are unintentional oil additives, but most large metal particles and impurities are removed in situ using either magnets or oil filters made for this purpose. See tribology, the science that studies how materials wear.
