Motor oil, or engine oil, is an oil used for lubrication of various internal combustion engines. While the main function is to lubricate moving parts, motor oil also cleans, inhibits corrosion, improves sealing and cools the engine by carrying heat away from the moving parts. The majority of motor oils are derived from petroleum. Motor oil mostly consists of hydrocarbons, organic compounds consisting entirely of hydrogen, and carbon.
Lubricating oil creates a separating film between surfaces of adjacent moving parts to minimize direct contact between them, decreasing friction, wear, and production of excessive heat, thus protecting the engine. Motor oil also carries away heat from moving parts, which is important because materials tend to become softer and less abrasion-resistant at high temperatures. Some engines have an additional oil cooler for this purpose.
In petrol (gasoline) engines, the top compression ring can expose the motor oil to temperatures of 320 °F (160 °C). In diesel engines the top ring can expose the oil to temperatures over 600 °F (315 °C). Motor oils with higher viscosity indices thin less at these higher temperatures.
Coating metal parts with oil also keeps them from being exposed to oxygen, inhibiting oxidation at elevated operating temperatures preventing rust or corrosion. Corrosion inhibitors may also be added to the motor oil. Many motor oils also have detergent and dispersant additives to help keep the engine clean and minimize oil sludge build-up.
Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Sludge also accumulates in the engine. Such particles could circulate in the oil and grind against the moving parts, causing erosion and wear. Because particles inevitably build up in the oil, it is typically circulated through an oil filter to remove harmful particles. An oil pump, a vane or gear pump powered by the vehicle engine, pumps the oil throughout the engine, including the oil filter. Oil filters can be a full flow or bypass type.
In the crankcase of a vehicle engine, motor oil lubricates rotating or sliding surfaces between the crankshaft journals bearings (main bearings and big-end bearings), and rods connecting the pistons to the crankshaft. The oil collects in an oil pan, or sump at the bottom of the crankcase. In some small engines such as lawn mower engines, dippers on the bottoms of connecting rods dip into the oil at the bottom and splash it around the crankcase as needed to lubricate parts inside. In modern vehicle engines, the oil pump takes oil from the oil pan and sends it through the oil filter into oil galleries, from which the oil lubricates the main bearings holding the crankshaft up at the main journals and camshaft bearings operating the valves. In typical modern vehicles, oil pressure-fed from the oil galleries to the main bearings enters holes in the main journals of the crankshaft. From these holes in the main journals, the oil moves through passageways inside the crankshaft to exit holes in the rod journals to lubricate the rod bearings and connecting rods. Some simpler designs relied on these rapidly moving parts to splash and lubricate the contacting surfaces between the piston rings and interior surfaces of the cylinders. However, in modern designs, there are also passageways through the rods which carry oil from the rod bearings to the rod-piston connections 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 chamber in the cylinder head from the crankcase. The oil then drips back down into the oil pan. .
Other non-motor oils include gear or transmission, and differentials oils. These are used in manual gearboxes and driven axles. They could include specialty uses including EP (Extreme Pressure), hypoid, and limited slip functions. Again, they are not to be used for engine lubrication.
In addition to the 2-cycle oil used if they have gasoline engines, chain saws also separately use "bar and chain oil" for lubricating the surfaces where the cutting chain moves around bar.
Other examples of mechanical equipment often using oil include oil-driven compressors, vacuum pumps, diffusion pumps, sewing machines and other devices with motors, oil-driven hydraulic equipment, and turbines.
The oil properties will vary according to the individual needs of these devices.
Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives to improve certain properties. One of the most important properties of motor oil in maintaining a lubricating film between moving parts is its viscosity. 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.
Motor oil must be able to flow at cold winter temperatures to lubricate internal moving parts upon starting up the engine. 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 vapors which can ignite. It is dangerous for the oil in a motor to ignite and burn, so a high flash point is desirable. At a petroleum refinery, fractional distillation separates a motor oil fraction from other crude oil fractions, removing the volatile components which ignite more easily, and therefore increasing the oil's flash point.
Another test done on oil is to determine the Total Base Number (TBN), which is a measurement of the reserve alkalinity of an oil to neutralize acids. The resulting quantity is determined as mg KOH/ (gram of lubricant). Analogously, Total Acid Number (TAN) is the measure of a lubricant's acidity. Other tests include zinc, phosphorus, or sulfur content, and testing for excessive foaming.
Different motor oils are sold for Diesel fuel engines, with many claimed to contain a higher level of detergents and dispersants to keep fine combustion soot in suspension. However, for some brands only the packaging varies (the oil is the same), and in general a diesel engine can use any good quality oil of the correct grade and specification.
The NOACK volatility (ASTM D-5800) Test determines the evaporation loss of lubricants in high temperature service. A maximum of 15 percent evaporation loss is allowable to meet API SL and ILSAC GF-3 specifications.
The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. SAE viscosity gradings include the following: 0, 5, 10, 15, 20, 25, 30, 40, 50 or 60. Some of the numbers can be suffixed with the letter W, designating their "winter" or cold-start viscosity, at lower temperature.
Viscosity is graded by measuring the time it takes for a standard amount of oil to flow through a standard orifice, at standard temperature. The longer it takes, the higher the viscosity, and thus higher SAE code.
Note that the SAE operate a separate viscosity rating system for transmission oils which should not be confused with engine oil viscosity. The higher numbers of a transmission oil (eg 75W-140) do not mean that it is necessarily higher viscosity than an engine oil.
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, and vintage or classic cars.
The SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. Historically, the first number associated with the W (again 'W' is for Winter, not Weight) is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as well as a single-grade SAE 10 oil can be pumped. "5W" can be pumped at a lower temperature than "10W" and "0W" can be pumped at a lower temperature than "5W". The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. This "classic" method of defining the "W" rating has since been replaced with a more technical test where a "cold crank simulator" is used at increasingly lowered temps. A 0W oil is tested at , a 5W at and a 10W is tested at . The real-world ability of an oil to crank in the cold is diminished soon after put into service. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle (although some modern European cars now make no viscosity requirement), but can vary from country to country when climatic or mpg constraints come into play.
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, an often-misunderstood phenomenon.
Note that the API oil classification structure has eliminated specific support for wet-clutch motorcycle applications in their descriptors, and API SJ and newer oils are referred to be specific to automobile and light truck use. Accordingly, motorcycle oils are subject to their own unique standards.
The latest API service standard designation is SM for gasoline automobile and light-truck 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, SL and SJ for gasoline engines. All previous service designations are obsolete, although motorcycle oils commonly still use the SF/SG standard. The obsolete SH standard was the last standard to contain the integral zinc and phosphorus (ZDDP) levels needed for proper lubrication of approx. pre-1990 cars. Oils with higher ZDDP levels are still available from some manufactures, although much information is proprietary.
There are seven diesel engine service designations which are current: CJ-4, CI-4 Plus, CI-4, CH-4, CG-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 may display the API starburst symbol with the service designation on containers sold to oil users.
The IIIG test is about 50% more difficult than the previous IIIF test, used in GF-3 and API SL oils. Engine oils bearing the API starburst symbol since 2005 are ILSAC GF-4 compliant.
For 4-stroke gasoline engines, the JASO T904 standard is used, and is particularly relevant to motorcycle engines. The JASO T904-MA and MA2 standards are approved wet clutch use, and the JASO T904-MB standard is not suitable for wet clutch use.
For 2-stroke gasoline engines, the JASO M345 (FA, FB, FC) standard is used, and this refers particularly low ash, lubricity, detergency, low smoke and exhaust blocking.
These standards, especially JASO-MA and JASO-FC are designed to address oil-requirement issues not addressed by the API service categories.
Probably the most well known of these are the VW50*.0* series from Volkswagen Group, and the MB22*.** from Mercedes-Benz. Other European OEM standards are from General Motors, for the Vauxhall, Opel and Saab brands, the Ford "WSS" standards, BMW Special Oils and BMW Longlife standards, Porsche, and the PSA Group of Peugeot and Citroën.
In recent times, very highly specialised "extended drain" "longlife" oils have arisen, whereby, taking Volkswagen Group vehicles, a petrol engine can now go up to 2 years or 30,000 km (a little under 20,000 miles), and a diesel engine can go up to 2 years or 50,000 km (a little under 30,000 miles) - before requiring an oil change. BMW, GM, Mercedes and PSA all have their own similar longlife oil standards.
Furthermore, virtually all European OEM standards require a long duration of longevity of the HTHS (High Temperature, High Shear) viscosity, many around the 3.5 cP.
As a result of this ultra-modern development in oil technology, and the subsequent development of the engines themselves (particularly with powerful engine electronic ECUs), virtually all modern European cars will demand a specific OEM-only oil standard. As a result, they now invariably make no reference at all to API standards, nor SAE viscosity grades. They may also make no primary reference to the ACEA standards, with the exception of being able to use a "lesser" ACEA grade oil for "emergency top-up", though this usually has strict limits, often up to a maximum of ½ a litre of non-OEM oil.
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:
Instead of making motor oil with the conventional petroleum base, "true" synthetic oil base stocks are artificially synthesized. Synthetic oils are derived from either Group III mineral base oils, Group IV, or Group V non-mineral bases. True synthetics include classes of lubricants like synthetic esters as well as "others" like GTL (Methane Gas-to-Liquid) (Group V) and polyalpha-olefins (Group IV). Higher purity and therefore better property control theoretically means synthetic oil has good mechanical properties at extremes of high and low temperatures. The molecules are made large and "soft" enough to retain good viscosity at higher temperatures, yet branched molecular structures interfere with solidification and therefore allow flow at lower temperatures. Thus, although the viscosity still decreases as temperature increases, these synthetic motor oils have a much improved viscosity index over the traditional petroleum base. Their specially designed properties allow a wider temperature range at higher and lower temperatures and often include a lower pour point. With their improved viscosity index, true synthetic oils need little or no viscosity index improvers, which are the oil components most vulnerable to thermal and mechanical degradation as the oil ages, and thus they do not degrade as quickly as traditional motor oils. However, they still fill up with particulate matter, although at a lower rate compared to conventional oils, and the oil filter still fills and clogs up over time. So, periodic oil and filter changes should still be done with synthetic oil; but some synthetic oil suppliers suggest that the intervals between oil changes can be longer, sometimes as long as 10,000 - 15,000 miles.
With improved efficiency, synthetic lubricants are designed to make wear and tear on gears far less than with petroleum-based lubricants, reduce the incidence of oil oxidation and sludge formation, and allow for "long life" extended drain intervals. Today, synthetic lubricants are available for use in modern automobiles on nearly all lubricated components, potentially with superior performance and longevity as compared to non-synthetic alternatives. Some tests have shown that fully synthetic oil is superior to conventional oil in many respects, providing better engine protection, performance, and better flow in cold starts than petroleum-based motor oil.
The vehicle manufacturer may specify which SAE viscosity grade of oil should be used for the vehicles it produces, but many different weights can actually be used. Some manufacturers have specific quality test requirements or "specs" for service in their particular make. In the USA, most quick oil change shops recommended intervals of 3,000 miles or every 3 months.
With a degree of ambiguity about how many miles motor oil is actually good for, some people opt for a more convenient time-based schedule. Seasonal changes are desirable where the viscosity can be adjusted for the ambient temperature change, thicker for summer heat and thinner for the winter cold. As a general rule, the thinnest oil that does not produce excess wear is used. Time-based intervals account for both the short trip driver who does fewer miles, but builds up more contaminates, as well as the long highway trips that are much easier on the oil. Many modern cars now list somewhat higher intervals for changing of oil and filter, with the constraint of "severe" service requiring more frequent changes with less-than ideal driving. Most commonly this applies to short trips of under 10 miles, where the oil does not get to full operating temps long enough to burn off condensation, excess fuel, and other contamination that leads to "sludge", "varnish", "acids", or other deposits. Many manufacturers have engine computer calculations to estimate the oil's condition based on the factors which degrade it such as RPMs, temperatures, and trip length; and one system adds an optical sensor for determining the clarity of the oil in the engine. These systems are commonly known as Oil Life Monitors or OLMs. Over the years, manufacturers have been able to reduce the viscosity of oil needed to correctly lubricate the engine and extend the duration of the servicable life. In the 1970s, typical cars took heavy 10W-40 oil which was used for a duration of 2000 miles or less. In the 1980s, 5W-30 oils were introduced to improve gas mileage and engine performance. A modern typical application would be Honda Motor's use of 5W-20 viscosity oil for 7500 miles without excess wear or deposits, while offering maximum mpg. Most other manufacturers use 20-weight oils as well. The latest API "SM" spec offers a substantially better product than preceding specifications.
New Biodegradable Auto Oil is making an appearance on the market. This oil is formed from the fats of cattle. The benefit of this new form of motor oil is its ability to get back into soil with fewer negative consequences. Typical motor oil needs to go through special treatment facilities, whereas biodegradable motor oil has less impact on the environment if spilled on the ground. All used motor oils can contain toxic heavy metals, however, and even biodegradable oils should be recycled properly.