Most Commonly Used Additives

Anti-wear Agents - In highly loaded parts like camshafts and valve lifters, an oil film is usually squeezed so thin it breaks. This would mean damaging metal-to-metal contact. When the oil film breaks down, these agents react chemically with the metal surfaces and form a protective coating that reduces wear.
Detergent-Dispersants - Combustion contaminants and dirt particles in the oil tend to clump, which can lead to sludge and varnish deposits. Detergent-dispersants are the additives that not only keep the engine clean but also stop such particles from massing. They work by surrounding the particles, most of which are invisible, with a shell of molecules. This action causes the imprisoned particles to repel one another. It also makes them a part of the oil, like cream is part of homogenized milk. They cannot settle out on engine parts or clog oil passages.
Oxidation Inhibitors - These are additives that allow oil to lubricate when it's hot.
Oxidation is a complex chemical reaction between oil molecules and oxygen. As the operating temperature of an oil goes up, so does the oil's oxidation rate. And unless an oil is heavily enriched with additives to fight oxidation, all sorts of undesirable things could happen. The worst of these is that the oil gets thicker and thicker - permanently.
Thus, a driver who waits too long between oil changes or uses low-quality oil and embarks on a cross-country trip during the summer may find the oil oxidized to a point so thick it won't drain out of the crankcase.
Rust and Corrosion Inhibitors - For every gallon of gasoline burned, about a gallon of water forms as steam. Most of the steam goes out the tailpipe. But some of it leaks as part of the blowby gases into the crankcase, where it becomes acidic. Water build-up in the oil increases with short-trip driving. If it weren't for rust and corrosion inhibitors and regular oil changes, engines would wear because of that water.
V.I. Improvers - V.I. stands for viscosity index. Oils having a high viscosity index tend to thin out less when heated and thicken less when cooled. V. I. improvers are used to raise an oil's viscosity index for all-season operation. Without them there wouldn't be multigrade oils.
Foam Inhibitors - Foam can result from air being whipped into the oil by moving engine parts, which could lead to oil loss, hydraulic valve lifter noise, and improper cooling and lubrication. Foam inhibitors weaken the surface tension of oil, which causes the air bubbles to break more readily.
Pour Point Depressants - The pour point of an oil is the lowest temperature at which the oil will flow. Below that temperature, wax crystals form, grow and interconnect, then solidify or "freeze" the oil. Pour point depressants lower this freezing temperature by coating the wax crystals as they form, thereby checking crystal growth.
Friction Modifiers - These are the newest members of the family of additives in modern oils. They further reduce surface friction between moving parts. Friction modifiers, along with low viscosity, make "Energy Conserving" and similarly labeled oils possible. Such oils can increase a vehicle's fuel economy by several percent.