There was a time when engine oil was simply engine oil. During the 1930s, oil manufacturers began to add wax modifiers to base oil to deal with wax residue. Today most motor oils contain many additives and after-oil additives and treatments are also available. There are three main purposes for using oil additives, protecting the surfaces of engine parts, improving the performance of the oil and extending the life of the oil.
Pure base oil has a single weight or viscosity. Viscosity is the resistance of a fluid to flow. The higher the viscosity number, the more resistant the oil is to flow, or the thicker it is. Multi-grade oils have additives referred to as viscosity index improvers. These additives cause multi-grade oils with viscosities or weights of 10W-40 to be no thicker than 10-weight oil at zero degrees Fahrenheit and no thinner than a 40 weight oil at 212 degrees. There are two disadvantages to viscosity index improves. The first is that they are not lubricants. The bigger the difference in the two numbers in multi-grade oils, the more viscosity index improvers there are in the oil, which means there is less actual oil. The second disadvantage to viscosity index improvers is that they tend to break apart under the pressure of being forced through a running engine. With each broken VII molecule, the oil loses a little of its viscosity at high temperatures.
The advantages of some engine oil additives is that they protect the metal surfaces of the engine, especially when the oil's lubricating film breaks down. Anti-wear agents protect against friction and engine seizure. Zinc and phosphorus compounds are often used as anti-wear agents. Others include phosphorus and sulfur compounds. A disadvantage of sulfur additives is that they can adversely affect fuel economy and damage catalytic converters. Detergents in engine oil inhibit rust and deposits and remove some solid deposits. Dispersants hold solid particles in suspension so that they don't form sludge. Some additives serve both the functions of detergents and dispersants. Some detergent additives can work against other additives used to reduce friction. Certain detergent and dispersant additives can induce foaming and formation of deposits at high temperatures.
Viscosity index improvers fall into this category of additives. Even with the use of VIIs, engine oil contains wax that can cause oil to thicken, so pour point depressants are added to the oil to keep the wax from congealing, allowing it to pour and flow more easily at colder temperatures.
Oil tends to foam as the crankshaft spins through it. Foam does not lubricate as effectively as liquid oil, so foam inhibitors are added which reduce surface tension to collapse the bubbles. Too much foam inhibitor additive can have the opposite effect and promote foaming. With modern engines operating at ever higher temperatures, there is the danger of engine oil oxidizing and thereby thickening. Antioxidant additives slow or prevent this process by decomposing peroxides.
