Use of additives is an approach to improve and maintain oil performance. Because high engine temperatures will combine with moisture, combustion by-products, rust, corrosion, engine wear particles and oxygen, sludge and varnish will be produced. The role of oil additives will not only assist in maintaining good lubrication, but also help minimize sludge and varnish, and any damage that would occur to your engine as a result.
There are hundreds of oil additives on the market. Some say that they will reduce your mileage, or reduce your wear, or reduce your oil consumption, and some even say that you can run your engine without any oil after treating your engine with their miracle cure additive.
Keep in mind that the base oil package in any oil can range from 70-95% of the mix, the rest comprised of additives. However, since some base oils will actually have natural characteristics which reduces the need for additional additives, comparing % of additives between different oils doesn't mean anything.
The ingredients in an additive package differ in cost, but price is just one factor. Some work better in certain combinations of base oils, and some of the less-expensive base oils are a good choice for a blend because of the way they perform with popular additives. Bottom line: every motor oil has a recipe. Refiners come up with a list of objectives based on the needs of their customers (vehicle manufacturers) and formulate oil to meet those goals as best they can.
DetergentsThe purpose is to keep the surfaces of your engine parts clean by inhibiting the formation of high-temperature deposits, rust and corrosion.
DispersantsThese additives disperse solid particles, keeping them in solution, so they don't come together to form sludge, varnish and acids. Some additives work both as detergents and dispersants.
Antiwear AgentsThere are times when the lubricating film breaks down, so the antiwear agents have to protect the metal surfaces. A zinc and phosphorus compound called ZDDP along with other phosphorus (and sulphur) compounds are commonly used.
Friction ModifiersThese reduce engine friction and therefore can improve fuel economy. Graphite, molybdenum and other compounds are used.
Pour-Point DepressantsOil contains wax particles that can congeal and reduce flow, so these additives are used to prevent that occurring.
AntioxidantsThis prevents oxidation (thickening) of oil. Some of the additives that perform other functions also serve this purpose, such as the antiwear agents.
Foam InhibitorsThe crankshaft whipping through the oil in the pan causes foaming. Oil foam is not as effective a lubricant as a full-liquid stream, so the inhibitors are used to cause the foam bubbles to collapse.
Rust/Corrosion InhibitorsUsed to protect metal parts from acids and moisture.