Motor oil is an essential part of internal combustion engines, designed not only to lubricate moving parts, but to clean and cool the engine, inhibit corrosion, and improve sealing.   

Motor oil is primarily made up of petroleum, with most made up from hydrocarbon stock rendered from crude oil. Though petroleum based motor oil is the most common, synthetic lubricants that can be used in much the same way, and on the majority of the same components as motor oil, are also available.  One of the most important factors that allow motor oil to lubricate moving parts is its viscosity.  The viscosity of motor oil must allow for a delicate balance – it must be thick enough that it coats moving parts, but agile enough that it maintains a lubricating film as parts move together.  Temperature can affect the viscosity index of motor oil, so it is important that a user is aware of the viscosity of the oil chosen for his or her task.   

Motor oil is an essential component of equipment such as motor vehicles, large construction equipment, trains and aircraft, and electrical generators.  When used as a lubricant, it acts as a barrier between moving parts in an internal combustion engine. When parts are not properly lubricated, they move against one another and cause friction. Friction dispels itself as heat, but wastes useful power that would have otherwise been generated if the part had been properly lubricated with motor oil.  Contact between those moving parts also causes engine wear, but the proper usage of motor oil can reduce wear and increase the performance and life of the engine. Aside from acting as a lubricant, motor oil also carries heat away from moving parts. 

Motor oil can also help keep and engine clean by combating corrosion.  Coating metal parts with motor oil keeps them from being exposed to oxygen at elevated operating temperatures.  Often, corrosion inhibitors and other deterrents against dirt and sludge build up are added to motor oil.   

Though motor oil is often thought of as exclusive to use in vehicles, equipment such as lawn mowers, leaf blowers, chain saws, electrical generators and even some model airplanes also require motor oil. Fortunately, since these machines are not exposed to the types of applications that vehicle motors generally deal with, they may be a single grade or have less viscosity index improver than types of motor oil used in vehicles.