Oil additives are chemical compounds that improve the lubricant performance of base oil (or oil "base stock"). The manufacturer of many different oils can utilize the same base stock for each formulation and can choose different additives for each specific application. Additives comprise up to 5% by weight of some oils.
Nearly all commercial motor oils contain additives, whether the oils are synthetic or petroleum based. Essentially, only the American Petroleum Institute (API) Service SA motor oils have no additives, and they are therefore incapable of protecting modern engines. The choice of additives is determined by the application, e.g. the oil for a diesel engine with direct injection in a pickup truck (API Service CJ-4) has different additives than the oil used in a small gasoline-powered outboard motor on a boat (2-cycle engine oil).
Types of additives
Oil additives are vital for the proper lubrication and prolonged use of motor oil in modern internal combustion engines. Without many of these, the oil would become contaminated, break down, leak out, or not properly protect engine parts at all operating temperatures. Just as important are additives for oils used inside gearboxes, automatic transmissions, and bearings. Some of the most important additives include those used for viscosity and lubricity, contaminant control, for the control of chemical breakdown, and for seal conditioning. Some additives permit lubricants to perform better under severe conditions, such as extreme pressures and temperatures and high levels of contamination.
Controlling chemical breakdown
- 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. Typical detergents are magnesium sulfonates.
- Corrosion or rust inhibiting additives retard the oxidation of metal inside an engine.
- Antioxidant additives retard the degradation of the stock oil by oxidation. Typical additives are organic amines and phenols.
- Metal deactivators create a film on metal surfaces to prevent the metal from causing the oil to be oxidized.
- Viscosity modifiers make an 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. Viscosity modifiers are often plastic polymers.
- Pour point depressants improve the oil's ability to flow at lower temperatures.
- Friction modifiers or friction reducers, 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.
- Extreme pressure agents bond to metal surfaces, keeping them from touching even at high pressure.
- Antiwear additives or wear inhibiting additives cause a film to surround metal parts, helping to keep them separated. Zinc dialkyldithiophosphate  or zinc dithiophosphates are typically used.
- 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. Tribology is the science that studies how materials wear.
For contaminant control
- Dispersants keep contaminants (e.g. soot) suspended in the oil to prevent them from coagulating.
- Anti-foam agents (defoamants) inhibit the production of air bubbles and foam in the oil which can cause a loss of lubrication, pitting, and corrosion where entrained air and combustion gases contact metal surfaces.
- Antimisting agents prevent the atomization of the oil. Typical antimisting agents are silicones.
- 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.
For other reasons
Additives in the aftermarket and controversy
Although motor oil is manufactured with numerous additives, aftermarket oil additives exist, too. A glaring inconsistency of mass-marketed aftermarket oil additives is that they often use additives which are foreign to motor oil. On the other hand, commercial additives are also sold that are designed for extended drain intervals (to replace depleted additives in used oil) or for formulating oils in situ (to make a custom motor oil from base stock). Commercial additives are identical to the additives found in off-the-shelf motor oil, while mass-marketed additives have some of each.
Some mass-market oil additives have caused a major backlash by consumers and the U.S. Federal Trade Commission which investigated many mass-marketed engine oil additives in the late 1990s.
Although PTFE, a solid, was used in some aftermarket oil additives, users alleged that the PTFE clumped together, clogging filters. Certain people in the 1990s have reported that this was corroborated by NASA and U.S. universities. One thing to note, in defense of PTFE, is that if the particles are smaller than what was apparently used in the 1980s and 1990s, then PTFE can be an effective lubricant in suspension. The size of the particle and many other interrelated components of a lubricant make it difficult to make blanket statements about whether PTFE is useful or harmful. Although PTFE has been called "the slickest substance known to man", it would hardly do any good if it remains in the oil filter.
- The American Chemistry Council's Petroleum Additives Panel link to oil additive manufacturers.
- Thorsten Bartels et al. "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Weinheim. doi:10.1002/14356007.a15_423
- API's Engine Oil Guide, 2006
- Chevron Oronite's Diesel Additives
- Roger F. Sebenik et al. "Molybdenum and Molybdenum Compounds" in Ullmann's Encyclopedia of Chemical Technology 2005; Wiley-VCH, Weinheim. doi:10.1002/14356007.a16_655
- Whale oil dexron Turbo hydra-matic 350 By Ron Sessions], page 20.
- A NASA research report is purported to say about PTFE oil additives, "In the types of bearing surface contact we have looked at, we have seen no benefit. In some cases we have seen detrimental effect. The solids in the oil tend to accumulate at inlets and act as a dam, which simply blocks the oil from entering. Instead of helping, it is actually depriving parts of lubricant." The source of this quote is unknown, but the quote itself appears in the magazine article referenced below.
- See Road Rider Magazine (now Motorcycle Consumer News) article from August 1992 by Fred Rau, which has been reprinted extensively, and see oilsfilters.htm for a contemporary discussion.
- See Nanoflon, a PTFE that is small enough for suspension in lubricants and used commercially for that purpose.
- Presenting PTFE: A Potent Resin, A Well-Kept Secret by Owen Heatwole, April 1981, for QMI.
- QMI Products ad