The purpose is to keep the surfaces of your engine parts clean by inhibiting the formation of high-temperature deposits, rust and corrosion.
These 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.
There 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.
These reduce engine friction and therefore can improve fuel economy. Graphite, molybdenum and other compounds are used.
Oil contains wax particles that can congeal and reduce flow, so these additives are used to prevent that occurring.
This prevents oxidation (thickening) of oil. Some of the additives that perform other functions also serve this purpose, such as the antiwear agents.
The 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.
Used to protect metal parts from acids and moisture.