Motor Oil Basics

What does motor oil actually do in an engine?

A lot! Motor oil probably handles more tasks than any other component, fluid or material in an engine. While most people know the motor oil's main job is to protect an engine by lubricating its moving parts, it actually does a lot more.

It cools critical engine parts by transferring heat away from them.
Seals and protects against wear cause by contaminants, whether from acidic combustion by-products or sludge and soot deposits, or even dirt and dust.
It needs to pump easily, even at low temperatures.
It needs to remain stable at higher temperatures.
It helps improve fuel economy.
Keeps internal component clean and free from varnish and harmful deposits.

What's inside the motor oil that enables it to perform all these tasks?

Since lubricating engine components (reducing friction for smoother running and optimal efficiency) is only part of what motor oil must do, it obviously must rely on help. That's where additives come in. Many motor oils contain detergent and dispersant additives that suspend contaminants and combustion by-products. That's why your oil may look dirty when you check it: it's doing its job. That's also why you can't tell when it's time to change your oil simply by looking at its color. The oil is designed to prevent these corrosive contaminants from being deposited on engine surfaces, where they can cause piston rings to sitck and oil-pump screens to plug. The oil filter helps by removing abrasive particles as the oil passes through the filter. However, an oil filter can't catch fuel that is diluting the oil or other liquid contaminants.

No matter how good the oil or the oil filter, eventually they are unable to do their jobs effectively. Time and mileage use up the oil's additives and cause it to degrade (meaning oxidize or thicken). At that point, the oil must be changed before sludge and deposits build up in the engine. When the oil is drained, the contaminants are removed with it. Obviously, oil should be changed before its contamination level reaches the point where engine damage could result. However, it's unlikely that the individual motorist will be able to determine this critical point. This is why automobile manufacturers recommend oil changes at a specific time or mileage interval, whichever comes first.

My owner's manual lists two different schedules. How do I know if I should follow the "normal" or "severe" service schedule?

Many manufacturers list two types of service, normal and severe. Normal (or ideal) service consists of relatively high speed driving (35-65 mph) on paved roads in dust-free areas, with trips over 10 miles each. The owner's manual uses this type of driving to determine your car's maximum time or mileage oil change recommendations. Severe service operations will require more frequent oil and filter changes. Severe service can include any of the following, among others:

Trips shorter than 10 miles
Driving in cold weather
Idling for extended periods
Stop and go traffic
Pulling trailers/carrying heavy loads
Driving in dusty conditions

Short trips can be particularly severe, especially during cold weather. The engine may not get a chance to warm up enough to boil off moisture and unburnt fuel in the oil. This leads to the development of acids in the oil. Since the engine is partially cooled by airflow, temperatures can increase when the engine is idling. High temperatures can lead to more rapid oil oxidation and thickening.

Credit: The content for this article was borrowed largely from a publication by Exxon Mobil Corporation.