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CATEGORIES (articles) > General > Fluids > Motor Oil

Motor Oil

Motor oil is a type of liquid oil used for lubrication by various kinds of motors, especially internal combustion engines. Other benefits from using motor oil include cooling by carrying heat away from moving engine parts and often include cleaning and corrosion inhibition in internal combustion engines. The major fraction of the majority of motor oils is derived from crude oil.

Use of motor oil

Motor oil is used for lubrication by various kinds of motors, especially internal combustion engines in vehicles, boats, lawn mowers, trains, some airplanes, etc. In motors, there are contacting parts which move against each other at high speeds, often for prolonged periods of time. Such rubbing motion causes friction, absorbing otherwise useful power produced by the motor and converting the energy to relatively useless heat. Friction also wears away the contacting surfaces of those parts, which could lead to lower efficiency and wearing out of the motor.

Lubricating oil makes a film between surfaces of parts moving against each other so as to minimize direct contact between them. This lubricating film decreases friction, wearing, and production of excessive heat between the moving parts. Also as a moving fluid, motor oil carries away heat building up at the surfaces of parts, because some heat is inevitably produced anyway due to friction from parts moving against each other or the oil film. Materials tend to become softer and less abrasion-resistant at high temperatures.

In the crankcase of a vehicle engine, the crankshaft, bearings, and bottoms of rods connecting the pistons to the crankshaft are submerged in oil for lubrication of these rapidly moving parts. Also, the rapid motion of these parts causes churning of the oil to splash and lubricate the contacting surfaces between the piston rings and interior surfaces of the cylinders. This oil film also serves as a seal between the piston rings and cylinder walls to separate the combustion volume in the cylinders from the space in the crankcase.

Coating metal parts with oil also keeps them from being exposed to oxygen, which inhibits their oxidation at elevated operating temperatures. Corrosion inhibitors may also be added to the motor oil. Many motor oils also have detergent additives to help keep the engine clean and minimize sludge build-up.

Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Sludge accumulates in the engine also. Such particles could circulate in the oil and grind against the moving parts, causing erosion and wear. Because undesired particles inevitably build up in the oil, the motor oil is circulated through an oil filter to remove harmful particles. An oil pump powered by the engine pumps the oil around and through the oil filter.

Automatic transmission fluid is a separate fluid used for vehicle's transmissions. It is typically coloured red to distinguish it from the motor oil and other fluids in the vehicle.

Properties of motor oil

Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives added as needed to improve the properties. One of the most important properties of motor oil in maintaining a lubricating film between moving parts is its viscosity. In layman's terms, the viscosity of a liquid can be thought of as its "thickness" or a quantity of resistance to flow. The viscosity must be high enough to maintain a satisfactory lubricating film, but low enough that the oil can flow around the engine parts satisfactorily to keep them well coated under all conditions. The viscosity index is a measure of how much the oil's viscosity changes as temperature changes. A higher viscosity index indicates the viscosity changes less with temperature than a lower viscosity index.

Another important property of motor oil is its pour point, which is indicative of the lowest temperature at which the oil could still be poured satisfactorily. The lower the pour point temperature of the oil, the more desirable the oil is when starting up at cold temperature.

Oil is largely composed of hydrocarbons which can burn if ignited. Still another important property of motor oil is its flash point, the lowest temperature at which the oil gives off vapours which can ignite. It is, of course, dangerous for the oil to ignite and burn, so a high flash point is desirable. At a petroleum refinery, fractional distillation to separate crude oil fractions removes the volatile components, which more easily ignite, from the motor oil fraction; therefore, increasing the oil's flash point.

Another test done on oil is Total Base Number (TBN), which is a measurement of reserve alkalinity of an oil for neutralizing acids. The resulting quantity is determined as mg KOH/ (gram of lubricant). Analogously, Total Acid Number (TAN) is the measurement of a lubricant’s acidity content. Other tests include zinc, phosphorus, or sulphur content, and testing for excessive foaming.

Grades of motor oil

Single-grade motor oil

The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. See the article on viscosity for details on kinematic viscosity. For single-grade oils, the kinematic viscosity is measured at a reference temperature of 100°C (212°F) in units of centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as an SAE number 0, 5, 10, 20, 30, 40, 50, or 60. The higher the viscosity, the higher the SAE grade number is. These numbers are often referred to as the weight of a motor oil. The reference temperature is meant to approximate the operating temperature to which motor oil is exposed in an engine.

The viscosity of single-grade oil derived from petroleum unimproved with additives changes considerably with temperature. As the temperature increases, the viscosity of the oil decreases logarithmically in a relatively predictable manner. On single-grade oils, viscosity testing can be done at cold winter ( W ) temperature (as well as checking minimum viscosity at 100°C or 212°F) to grade an oil as SAE number 0W, 5W, 10W, 15W, 20W, or 25W. A single-grade oil graded at the hot temperature is expected to test into the corresponding grade at the winter temperature; i.e. a 10 grade oil should correspond to a 10W oil. For some applications, such as when the temperature ranges in use are not very wide, single-grade motor oil is satisfactory; for example, lawn mower engines.

Multi-grade motor oil

The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather. The difference in viscosities for any single-grade oil is too large between the extremes of temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers are added to the oil. These additives make the oil a multi-grade motor oil. The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened. This slope representing the change with temperature depends on the nature and amount of the additives to the base oil. To compare the variation of viscosity on temperature in a multi-grade oil with single-grade oils, two reference points are used. One is at cold (Winter) temperature, 0°C (32°F), and the other is at the hot operating temperature, 100°C (212°F).

Accordingly, the SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 is the designation for a common multi-grade oil. The first number associated with the W, the 10, means that the viscosity of this multi-grade oil at winter temperature corresponds to the viscosity of a single-grade 10W oil at winter temperature. The second number, 30, means that the viscosity of this multi-grade oil at hot operating temperature corresponds to the viscosity of a single-grade 30 oil at the operating temperature. The controlling SAE standard is called SAE J300. The motor oil grade to be used in a given vehicle is specified by the manufacturer of the vehicle.

Turbine Motor Oil

Turbine motor oils are designed somewhat differently than reciprocating engine oils traditionally used in automobiles. Deposit control and corrosion are not significant issues when formulating a turbine oil, and the sheer stresses that turbine oils are exposed to are minimal in light of the fact that turbines are naturally balanced rotating machines unlike reciprocating engines. Turbine oils tend to be formulated in the 3-7 cST range at 100degC, and make extensive use of polyolester chemistries due to the high temperatures they must endure.

In most aviation gas turbine applications, peak lubricant temperatures are not reached during engine operation, but after shutdown, when heat has been able to migrate from the combustor cans and the compressors into the regions of the engine with lubricated bearings and gearboxes. The gas flow associated with running the turbine provides significant convective cooling that disappears when the engine is shut down; leaving residual heat that causes temperatures within the turbine to rise dramatically. a very misunderstood phenomena.

American Petroleum Institute

Motor oil used for vehicle engines is commonly called engine oil in American Petroleum Institute (API) documentation. Engine oil is used for the lubrication, cooling, and cleaning of internal combustion engines. Motor oil may be composed of a lubricant base stock only in the case of non-detergent oil, or a lubricant base stock plus additives to improve the oil's detergency, extreme pressure performance, and ability to inhibit corrosion of engine parts. Lubricant base stocks are categorized into five groups by the API. Groups I and II are composed of fractionally distilled petroleum. Group III base stock is composed of hydro cracked and distilled petroleum. Group IV base stock is composed of polyalphaolefins (PAOs). Group V is a catch all group for synthetic base stocks. Examples of group V include polyol esters, polyalkalene glycols (PAG oils), and perfluoropolyalkylethers (PFPAEs). Groups I, II, and III are conventionally referred to as mineral oils and groups IV and V as synthetic oils, however certain aggressive marketers have recently labelled group III based oil as synthetic in the US. Caveat emptor.

Motor oils are further categorized by their API service class. The API service classes have two general classifications: S for Service (typical passenger cars and light trucks using gasoline engines) and C for commercial applications (typical diesel equipment). The latest API service standard designation is SM for gasoline engines. The SM standard refers to a group of laboratory and engine tests, including the latest series for control of high-temperature deposits. Current API service categories include SM, SJ and SL for gasoline engines. All previous service designations are obsolete. There are six Diesel engine service designations which are current: CI-4, CH-4, CG-4, CF-4, CF-2, and CF. All others are obsolete. It is possible for an oil to conform to both the gasoline and diesel standards. Engine oil which has been tested and meets the API standards has the API starburst symbol with the service designation on containers sold to oil users.

The International Lubricant Standardization and Approval Committee (ILSAC) also have standards for motor oil. Their latest standard, GF-4 was approved in 2004.


In engines, there is inevitably some exposure of the oil to products of internal combustion, and microscopic coke particles from black soot accumulate in the oil during operation. Also the rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against the part surfaces causing erosion and wear. The oil filter removes many of the particles, but eventually the oil filter gets filled up. The motor oil and especially the additives also undergo thermal and mechanical degradation. For these reasons, the oil and the oil filter need to be periodically replaced.

The vehicle manufacturer specifies which grade of oil should be used for the vehicles it produces. The manufacturer also specifies how often the oil changes should be made. For example, a common oil change frequency could be every 3000 miles or every 3 months, whichever comes sooner. For convenience, the oil filter is usually also replaced at the time the oil is changed. There are many types (or sizes) of oil filters for vehicle engines. Vendors of oil filters have information on which type of oil filter is compatible with a given vehicle.

When an engine is not running, the oil collects in an oil pan at the bottom of the crankcase. There is an oil drain plug normally screwed into a drain hole at the bottom of the oil pan which is accessible from underneath the vehicle. To change the oil in a vehicle's engine, the drain plug is unscrewed to let the oil drain out of the oil pan. After the used oil drains out, the plug is screwed back into the drain hole. The removable oil filter can be unscrewed at this time, often with the help of an oil filter removal tool, and a new oil filter screwed back in after applying fresh oil to the sealing surface of the new filter. Then new oil is poured in through an otherwise capped opening at the top of the engine. For many cars, 4 to 5 quarts or litres of oil are needed to fill the engine. In the engine, there is a removable dipstick, accessible from above the engine, to check the oil level while engine is not running. (In contrast, automatic transmission fluid level is checked with a separate dipstick while the engine is running.) Traditionally, lubrication at various joints in the vehicle is also done at the time of an oil change. Mechanics often call this maintenance routine "oil change and lube."

Changing a vehicle's oil should lead to improved fuel efficiency, lower temperature, and less wear since friction is reduced. Because changing a vehicle's oil could be messy and inconvenient for the owner since he/she must drain from under the vehicle, vehicle owners often have mechanics change the oil for them because they can hydraulically lift the vehicle to easily access the oil pan. Many quick oil change shops have appeared in the USA to conveniently provide this service to owners. Many of these shops have rooms below ground level for a mechanic to access the underside of a vehicle. In the USA, used motor oil is brought to recycling centres instead disposing it to avoid contaminating the environment.

History of Motor Oils

The history of using lubricants can be traced back as far back as 4,000 years ago. To build the pyramids, the ancient Egyptians used lubricants to move the large blocks of stone that made up the pyramids. Synthetic lubricants were invented initially for high temperature gas turbine/jet engine applications where traditional mineral-derived lubricants provided inadequate performance. In the mid 1970’s, synthetic motor oils were formulated and commercially applied for the first time in automotive applications. Improving the efficiency of lubricants, synthetic lubricants made wear and tear on gears far less then on the former petroleum based lubricants, reduced the incidence of oil oxidation and sludge formation, and allowed for extended drain intervals. Today, synthetic lubricants are used in modern day automobiles to lubricate and nearly all lubricated components often with superior performance and longevity compared to non-synthetic alternatives.

Other additives

In addition to the viscosity index improvers, motor oil manufacturers often include other additives such as detergents and dispersants to help keep the engine clean by minimizing sludge build-up, corrosion inhibitors, and alkaline additives to neutralize acidic oxidation products of the oil. Most commercial oils have a minimal amount of zinc dialkyldithiophosphate as an anti-wear additive to protect contacting metal surfaces with zinc in case of metal to metal contact. The quantity of zinc dialkyldithiophosphate is limited to minimize adverse effect on catalytic converters.

There are other additives available commercially which can be added to the oil by the user for purported additional benefit. Some of these additives include:

Zinc dialkyldithiophosphate additives are available to consumers for additional protection under extreme-pressure conditions or in heavy duty performance situations.
In the 1980s and 1990s, additives with suspended PTFE particles were available to consumers to increase motor oil's ability to coat and protect metal surfaces.
Some consumer additives are basically detergents to clean and dissolve sludge and/or carbon deposits.
Some molybdenum-containing additives to lubricating oils are claimed to reduce friction, bond to metal, or have anti-wear properties.


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