Motor oil has come a long way from very humble beginnings. In the early days of automobiles, an engine's parts were lubricated with simple, greasy byproducts left over from processing crude oil. This required very frequent service at an auto repair shop for oil changes.
Since then, oil has evolved to become a blend of a base oil and a set of heavily engineered and formulated chemicals. Together, these substances form a finely calibrated, viscous fluid that's designed to not only lubricate a car engine, but also to help keep it clean and free of debris. It's also better at transferring heat from the engine to keep it running cool. That's why oil changes are much easier and required less frequently than just a few decades ago.
What's In a Quart of Oil?
The main ingredient of modern motor oil is called the base oil. The base oil constitutes up to 80% of the volume of of a can of motor oil. There are two main types of base oil: petroleum and synthetic. Motor oil can use all of one type, or a blend of the two. An oil change can be performed with either type of oil.
Petroleum Oil
Oil made from petroleum base is similar to what was used years ago. Crude oil is broken down, cleaned of impurities, and treated to create lubricating oil. This oil is further processed and broken down into different grades by viscosity -- a measurement of how thick the oil is. Engines that run at higher temperatures and pressures require higher viscosities to avoid a loss of lubrication. Oil with lower viscosity is useful for engines used in cold climates. If you see a "W" after the viscosity rating on the label, that indicates that the oil is made for winter conditions.
Synthetic Oil
Synthetic motor oils use a man-made base oil, created from simple chemical building blocks. These purified chemicals are processed and combined to form new larger oil molecules. As a result, the resulting base oil is superior in quality, with far fewer impurities, than petroleum-based oil. Synthetic oils are generally more expensive than oils derived from crude oil, but they last longer and offer higher performance.
Not Just Oil
Beyond the base oil, modern motor oils also contain a set of chemicals known as a Detergent Inhibitor (DI) package. This very heavily engineered set of chemicals is what gives modern motor oil its amazing performance compared to older blends. These chemicals make the need for oil changes less frequent, and their superior performance has saved drivers from many trips to the auto mechanic. A DI package usually contains many substances:
Dispersants
These chemicals help oil trap and suspend particles that would otherwise be deposited in the engine. Thanks to dispersants, engines run cleaner and generally need fewer oil changes than cars of the past.
Detergents
Detergents also keep your engine clean, but they work differently than dispersants. Instead of simply keeping unwanted particles in suspension, detergents neutralize acids formed during combustion. These acids can damage the an engine, and lead to other deposits that form at high temperatures.
Friction Modifiers
These chemicals do exactly as they sound. They are used to help improve oil's ability to lubricate an engine. These chemicals have the added benefit of improving fuel economy, and they also help extend the life of the oil. This results in fewer oil changes without resulting in increased wear and tear on your engine -- or your wallet.
Seal Conditioners
Most oil will contain an additive that helps keep rubber seals and gaskets pliable. Since both petroleum-based and synthetic motor oil has a tendency to shrink and harden rubber seals, this additive is crucial to avoiding leaks that could lead to a loss of oil and an increase in wear.
Antioxidants
Oxidization is the reaction that molecules have when they come in contact with oxygen. The most common form of oxidation is rust on exposed metal parts, but there are many other kinds. Motor oil will thicken as it oxidizes, which affects the ability of the engine to circulate it properly. If left unchecked, oxidization leads to excessive carbon deposits inside the engine.
Zinc Dialkyldithiophosphate
Also known as ZDDP, manufacturers have used this chemical successfully for decades as an anti-wear and anti-oxidant additive. This additive is effective in reducing engine wear and oxidization, especially in the extreme pressure environment of high-performance car engines.
Rust & Corrosion Inhibitors
These chemicals are used to protect metals such as iron from corrosion. This corrosion can come from oxygen, water, or the acids that build up naturally as your engine operates at high temperatures.
Anti-Foaming Agents
The formation of bubbles in motor oil can cause multiple problems with performance and wear. Entrapped bubbles carry oxygen and other gases that increase oxidation on the surfaces they touch. They also interfere with the formation of a thin but complete film of oil that engine components need to run smoothly.
Diluent Oil