Lubricant Additives

A large number of additives are used to impart performance characteristics to the lubricants. The main families of additives are
Antioxidants.
Oxidation is the general attack of the weakest components of the base oil by oxygen in the air.  It occurs at all temperatures all of the time but is accelerated at higher temperatures and by the presence of water, wear metals and other contaminants.  It ultimately causes acids (which produce corrosion) and sludge (which results in surface deposits and viscosity to increase) to form.  Oxidation inhibitors, as they are also called, are used to extend the operating life of the oil.  They are sacrificial additives that are consumed while performing their duty of delaying the onset of oxidation, thus protecting the base oil.  They are present in almost every lubricating oil and grease.  
Detergents.
Detergents perform two functions.  They help to keep hot metal components free of deposits (clean) and neutralize acids that form in the oil.  Detergents are primarily used in engine oils and are alkaline or basic in nature.   They form the basis of the reserve alkalinity of engine oils, which is referred to as the base number (BN).  They are typically materials of calcium and magnesium chemistry.  Barium-based detergents were used in the past but are rarely used now. Since these metal compounds leave an ash deposit when the oil is burned, they may cause unwanted residue to form in high-temperature applications.  Due to this ash concern, many OEMs are specifying low-ash oils for equipment operating at high temperatures.  A detergent additive is normally used in conjunction with a dispersant additive.  
Friction modifiers.
Friction modifiers are typically used in engine oils and automatic transmission fluids to alter the friction between engine and transmission components.  In engines, the emphasis is on lowering friction to improve fuel economy.  In transmissions, the focus is on improving the engagement of the clutch materials.  Friction modifiers can be thought of as anti-wear additives for lower loads that are not activated by contact temperatures.  
Viscosity index improvers.
Viscosity index improvers are very large polymer additives that partially prevent the oil from thinning out (losing viscosity) as the temperature increases.  These additives are used extensively when blending multi-grade engine oils such as SAE 5W-30 or SAE 15W-40. They are also responsible for better oil flow at low temperatures, resulting in reduction in wear and improved fuel economy.  In addition, VI improvers are used to achieve high-VI hydraulic and gear oils for improved start-up and lubrication at low temperatures. To visualize how a VI-improver additive functions, think of the VI improver as an octopus or coil spring that stays coiled up in a ball at low temperatures and has very little effect on the oil viscosity.  Then, as the temperature rises, the additive (or octopus) expands or extends its arms (making it larger) and prevents the oil from thinning out too much at high temperatures.  VI improvers do have a couple of negative features.  The additives are large (high molecular weight) polymers, which makes them susceptible to being chopped or cut up into small pieces by machine components (shearing forces).  Gears are notoriously hard on VI-improver additives.  Permanent shearing of the VI-improver additive can cause significant viscosity losses, which can be detected with oil analysis.  A second form of viscosity loss occurs due to high shearing forces in the load zone of frictional surfaces (e.g., in journal bearings).  It is thought that the VI-improver additive loses its shape or uniform orientation and therefore loses some of its thickening ability.  The viscosity of the oil temporarily drops within the load zone and then rebounds to its normal viscosity after it leaves the load zone.  This characteristic actually aids in the reduction of fuel consumption. There are several different types of VI improvers (olefin copolymers are common).  High-quality VI improvers are less susceptible to permanent shear loss than low-cost, low-quality VI improvers
Demulsifiers
Demulsifier additives prevent the formation of a stable oil-water mixture or an emulsion by changing the interfacial tension of the oil so that water will coalesce and separate more readily from the oil.  This is an important characteristic for lubricants exposed to steam or water so that free water can settle out and be easily drained off at a reservoir.
Emulsifiers
Emulsifiers are used in oil-water-based metal-working fluids and fire-resistant fluids to help create a stable oil-water emulsion.  The emulsifier additive can be thought of as a glue binding the oil and water together, because normally they would like to separate from each other due to interfacial tension and differences in specific gravity
Pour Point Depressants
The pour point of an oil is approximately the lowest temperature at which an oil will remain fluid.  Wax crystals that form in paraffinic mineral oils crystallize (become solid) at low temperatures.  The solid crystals form a lattice network that inhibits the remaining liquid oil from flowing.  The additives in this group reduce the size of the wax crystals in the oil and their interaction with each other, allowing the oil to continue to flow at low temperatures.