These chemicals, usually metallo-organic based, are designed to control deposits and keep engine components clean. They are able to clean up existing deposits in the engine, as well as disperse insoluble matter into the oil. Detergents control contamination resulting from high temperature operation. Over-based detergents also neutralize acidic contaminants from fuel sulfur, engine exhaust, oil oxidation and/or nitration.

These are usually ashless organic chemicals, which control contamination from low temperature operation. Both detergents and dispersants attach themselves to contaminant particles, such as soot or varnish and hold them in suspension, preventing sludge and deposit formation. The suspended particles, together with their additive carrier, are so small that they can pass harmlessly between moving surfaces and through oil filters. This contamination is removed from the engine when the oil is changed.

These agents reduce oxygen attack on the lubricant base oil to a minimum. Severely hydrotreated base oils have a superior response to these additives compared to solvent refined base oils. This results in an engine oil with high resistance to oil thickening and the build-up of corrosive acids, hence maintaining good oil flow properties and resistance to bearing corrosion.

Acids are produced by the combustion process and when an engine oil degrades with use. Unless rendered harmless by the engine oil, these acids can cause rapid deterioration of engine components. Corrosion inhibitors protect non- ferrous metals by coating them and forming a barrier between the parts and their environment. Rust Inhibitors protect iron/steel surfaces from oxygen attack, by forming a similar protective screen as mentioned above. Parts such as hydraulic lifters, push rods, etc. are prone to this type of corrosion.

These agents prevent wear due to seizure or scuffing of rubbing surfaces. Compounds such as zinc dialkyl-dithiophosphate (ZDDP) break down at microscopic hot spots and form a chemical film which eliminates metal-to-metal contact before it grows. Thus scuffing, galling and seizure are prevented.

Detergent and dispersant additives can facilitate aeration of an oil, which leads to foaming. This can reduce the lubricating ability of an oil and even interfere with oil pumping. Incorporation of a foam depressant controls this tendency by reducing surface tension to speed up the collapse of foam.

VI improvers are long chain polymers that can coil and uncoil in response to changes in temperature. They improve an oil’s ability to resist changes in viscosity with temperature (i.e. improve its viscosity index). At low temperatures they coil up into tight balls which do not significantly increase the oil’s resistance to flow (viscosity). However, at high temperatures, they uncoil into long chains which interweave and increase the oil’s viscosity. VI improvers must resist breakdown due to shear and high temperatures to ensure a long lasting effect.

Base oils in API Groups I, II and III contain hydrocarbons that tend to crystallize into waxy materials at low temperatures. Incorporation of a chemical which reduces the size/rate of wax crystal formation can give an oil better low temperature fluidity, hence a lower pour point. Severely hydrotreated base oils have a minimum of waxy materials, so they have superior response to these additives compared to solvent refined base oils.

Some oils contain friction-modifying chemicals, which can reduce the fuel consumption of an engine. These chemicals form a chemical or physically bonded film that reduces the friction between the lubricated engine parts.