Additives are chemical agents added to oils, fuels, and coolants to impart specific beneficial properties to the finished products. Additives create new fluid properties, enhance properties already present and reduce the rate at which undesirable changes take place in a fluid during service. The percentage of additives can range from as little as 1-2% in some general purpose industrial lubricants to over 30% in cylinder oils oils used in engines burning heavy fuels. Some additives have a somewhat competitive nature and an overtreatment of one additive can have a negative effect on the performance of other additives in the lubricant formulation. Additives can be classified into one of three main types or functions: surface protection, fluid protection, or performance improvement. Some additives have multi-functional properties. Commonly used additives are:

Metal Surface Protection
Detergents Prevent & remove deposits, Neutralize acids, and may be used to create ash deposits that can help protect valve seats in some engine designs Neutral or Overbased Metal (Ca, Mg, Ba) Compounds (sulfonates, phenates, carbonates) Polymeric detergents
Corrosion Inhibitors Prevent corrosive attack on metallic & alloy surfaces Organically active Sulfur, Phosphorus, Nitogen compounds such as phospites, metal salts of thiophosphoric acid, sulfurized waxes, terpenes
Rust Inhibitors Prevent formation of rust on metal surfaces by formation of surface film or neutralization of acids High base additives, sulfonates, phosphates, organic acids or esters, amines
Anti-Wear ( AW) additives Minimize wear caused by metal-to-metal contact during conditions of mild to medium boundary lubrication, by forming polar bonded sacrificial protective film on the metal surface ZDDP ( Zinc dialkyldithiophosphates) TCP ( Tricresyl-phosphates)
Extreme Pressure (EP) additives Minimize wear, prevent scuffing caused by metal-to metal contact during conditions of extreme stress Sufur, Chlorine & Phosphorus compounds
Solid additives Reduce friction & wear during slow,sliding motion of two metal surfaces Graphite, MoS₂ (molybdenum disulfide)
Oil ( Fluid) Protection
Oxidation Inhibitors Prevent or control oxidation of the oil by breaking the oxidation reaction chain, decomposing peroxides formed by oxidation, and/or deactivating metal surfaces that may act as catalysts ZDDP, aromatic amines, sulfurized products, hindered phenols
Anti-Foamants Reduce foam with surface tension modifiers that break up large surface bubbles or small entrained bubble reducers Silicones, organic polymers
Metal Deactivators Form surface films so that metals do not catalyze oil oxidation ZDDP, metal phenates, Organic Nitrogen compounds
Dispersants Keep oil degradation by products, and/or combustion related by products in small suspended state within the bulk oil by preventing agglomeration Succinimides, amines
Oil ( fluid) Performance Enhancement
Viscosity Index Improvers ( VII) Reduce the rate at which oil viscosity decreases with increasing temperature Polyisobutylene, methacrylate polymers, olefin copolymers
Pour Point Depressants Impede formation of large wax crystals Low molecular weight methacrylate polymers
Tackiness agents Increase adhesiveness of lubricants on metal surfaces, reduce run-off High molecular weight polymers, aluminium soaps of unsaturated fatty acids
Emulsifiers Promote formation of stable mixture (emulsion) of water & oil by changing interfacial tension Soaps of fatty acids, sulfonic & napthenic acids, certain animal & vegetable oils
Demulsifiers Prevent formation of stable oil/water mixture (emulsion) by lowering surface tension at oil/water interface so that water will easily separate Heavy metal soaps, alkaline earth sulfonates
Seal Swell Agents Promote slight swelling of seal material to counteract shrinking action of some highly paraffinic ( Gr II/Gr III) and PAO base oils Esters
Oiliness agents Lower friction under near-boundary conditions High molecular weight polar compounds such as fatty oils, ixidized waxes
Friction Modifiers Reduce or modify friction Long chained polar compounds ( amides, phosphates, phosphites, acids, etc), MoS₂
Other Additives
Anti-Microbial Agents Control or kill microbial growth ( fungi, bacteria) Biocides, fungicides,
Dyes Identification of oils