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For single non-winter grade oils, the kinematic viscosity is measured at a temperature of 100 °C (212 °F) in units of mm²/s or the equivalent older non–SI units, centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as SAE viscosity grade 20, 30, 40, 50, or 60. In addition, for SAE grades 20, 30, and 40, a minimum viscosity measured at 150 °C (302 °F) and at a high–shear rate is also required. The higher the viscosity, the higher the SAE viscosity grade is.

СFor 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, industrial applications, and vintage or classic cars.

Multi-grade. The temperature range the oil is exposed to in most vehicles

иthe difference in viscosities closer together, special polymer additives called viscosity index improvers, or VIIs are added to the oil. These additives are used to make the oil a multi–grade motor oil, however it is possible to have a multi–grade

can be wide, ranging from cold 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. A specific oil will have high viscosity when cold and a lower viscosity at the engine's operating temperature. The difference in viscosities for

most single–gradeбoil is too large between the extremes of temperature. To bring

oil without the use of VIIs. The idea is to cause the multi–grade oil to have the

viscosity of the base grade when cold and the viscosity of the second grade when

hot. This enables one type of oil to be generally used all year. In fact, when multi–

grades were initially developed, they were frequently described as all–season oil.

The viscosity of a multi–grade oil still varies logarithmically with temperature, but

with temperature dependsАon the nature and amount of the additives to the base oil. The SAE designation for multi–grade oils includes two viscosity grades; for

the slope representing the change is lessened. This slope representing the change

10W and 30, and all limitations placedДon the viscosity grades (for example, a 10W–30 oil must fail the J300 requirements at 5W). Also, if an oil does not contain

example, 10W–30 designates a common multi–grade oil. The two numbers used

are individually defined by SAE J300 for single–grade oils. Therefore, an oil

labeled as 10W–30 must pass the SAE J300 viscosity grade requirement for both

any VIIs, and can pass as a multi–grade, that oil can be labeled with either of the two SAE viscosity grades. For example, a very simple multi–grade oil that can be easily made with modern baseoils without any VII is a 20W–20. This oil can be

The real–world ability of an oil to crank or pump when cold is potentially diminished soon after it is 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 have no viscosity requirement), but can vary from country to country when climatic or fuel efficiency constraints come into play.

Turbine motor oils are designed somewhat differently than reciprocating engine oils traditionally used in automobiles. Deposit control and corrosion are not

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significant issues when formulating a turbine oil, and the shear 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 have the ISO VG range 32, 46, and 68 (cSt at 40 °C/104 °F), and make extensive use of diester, polyolester, polyalphaolefin and Group II as base stock due to the high temperatures they must withstand. Some jet turbine oils contain an amount of polyglycols. Varnish is the most problematic contaminant, which can only be

Сdetected accurately with the ultra–centrifuge test resulting in the «UC value».

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

иperformance standards for lubricants. Motor 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 corrosionАof engine parts. Lubricant base stocks are categorized into five groups by the API. Group I base stocks are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve certain properties such as oxidation resistance and to remove wax. Group II base stocks are composed of fractionally distilled petroleum that has been hydrocracked to further refine andДpurify it. Group III base stocks have similar characteristics to Group II base stocks, except that Group III base stocks have higher viscosity indexes. Group III base stocks are produced by further hydrocracking of Group II base stocks, or of hydroisomerized slack wax, (a byproduct of the dewaxing process). Group IV base stock are polyalphaolefins (PAOs). Group V is a catch–all group for any baseИstock not described by Groups I to IV. Examples of group V base stocks include polyol esters, polyalkylene glycols (PAG oils), and perfluoropolyalkylethers (PFPAEs). Groups I and II are commonly referred to as mineral oils, group III is typically referred to as synthetic (except in Germany and Japan, where they must not be called synthetic) and group IV is a synthetic oil. Group V base oils are so diverse that there is no catch–all description.

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.

Standards.бThe American Petroleum Institute (API) sets minimum

API service classes. The API service classes have two general classifications: S for «service» (originating from spark ignition) (typical passenger cars and light trucks using gasoline engines), and C for «commercial» (originating from compression ignition) (typical diesel equipment). Engine oil which has been tested and meets the API standards may display the API Service Symbol (also known as the « Donut) with the service designation on containers sold to oil users.

Note that the API oil classification structure has eliminated specific support for wet–clutch motorcycle applications in their descriptors, and API SJ and newer

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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

Сstill use the SF/SG standard.

All the current gasoline categories (including the obsolete SH), have placed limitations on the phosphorus content for certain SAE viscosity grades (the xW– 20, xW–30) due to the chemical poisoning that phosphorus has on catalytic иconverters. Phosphorus is a key anti–wear component in motor oil and is usually found in motor oil in the form of Zinc_dithiophosphate. Each new API category has placed successively lower phosphorus limits, and this has created a controversial issue of backwards compatibility with much older engines, especially engines withбsliding tappets. API, and ILSAC, which represents most of the world’s major automobile/engine manufactures, states API SM/ILSAC GF–4 is fully backwards compatible, and it is noted that one of the engine tests required for API SM, the Sequence IVA, is a sliding tappet design to test specifically for cam wear protection. However, not everyone is in agreement with backwards compatibility, and inАaddition, there are special situations, such as «modified» engines or fully race built engines, where the engine protection requirements are above and beyond API/ILSAC requirements. Because of this, there are specialty oils out in the market place with higher than API allowed phosphorus levels.

There are six diesel engine service designations which are current: CJ–4, CI– 4, CH–4, CG–4, CF–2, and CF. AllДothers are obsolete. In addition, API created a separated CI–4 PLUS designation in conjunction with CJ–4 and CI–4 for oils that meet certain extra requirements, and this marking is located in the lower portion of the API Service Symbol «Donut».

It is possible for an oil to conform to both the gasoline and diesel standards. In fact, it is the norm for all diesel rated engine Иoils to carry the «corresponding» gasoline specification. For example, API CJ–4 will almost always list either SL or SM, API CI–4 with SL, API CH–4 with SJ ... etc.

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 buildup, 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 and other compounds in case of metal to metal contact. The quantity of zinc dialkyldithiophosphate is limited to minimize adverse effect on catalytic converters. Another aspect for after–treatment devices is the deposition of oil ash, which increases the exhaust back pressure and reduces over time the fuel economy. The so–called «chemical box» limits today the concentrations of sulfur, ash and phosphorus (SAP).

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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 (ZDDP) additives, which typically also contain calcium sulfonates, are available to consumers for additional protection under extreme–pressure conditions or in heavy duty performance situations. ZDDP and calcium additives are also added to protect motor oil from oxidative breakdown and to prevent the formation of sludge and varnish deposits.

СIn the 1980s and 1990s, additives with suspended PTFE particles were available e.g. «Slick50» to consumers to increase motor oil's ability to coat and protect metal surfaces. There is controversy as to the actual effectiveness of these products as they can coagulate and clog the oil filters.

иSome molybdenum disulfide containing additives to lubricating oils are claimed to reduce friction, bond to metal, or have anti–wear properties. They were used in WWII in flight engines and became commercial after WWII until the 1990s. They were commercialized in the 1970s (ELF ANTAR Molygraphite) and are today stillбavailable (Liqui Moly MoS2 10 W–40, www.liqui–moly.de).

Synthetic oil and synthetic blends. Synthetic lubricants were first synthesized, or man–made, in significant quantities as replacements for mineral lubricants (and fuels) by German scientists in the late 1930s and early 1940s because of their lack of sufficient quantities of crude for their (primarily military) needs. A significant Аfactor in its gain in popularity was the ability of synthetic– based lubricants to remain fluid in the sub–zero temperatures of the Eastern front in wintertime, temperatures which caused petroleum–based lubricants to solidify due to their higher wax content. The use of synthetic lubricants widened through the 1950s and 1960s due to a property at the other end of the temperature spectrum, the ability to lubricate Дaviation engines at temperatures that caused mineral–based lubricants to break down. In the mid–1970s, synthetic motor oils were formulated and commercially applied for the first time in automotive applications. The same SAE system for designating motor oil viscosity also applies to synthetic oils.

Instead of making motor oil with the conventionalИpetroleum 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

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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 16,000–24,000 km (10,000–15,000 mi).

СWith 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 иautomobiles 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–basedбmotor oil.

Maintenance. 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 particlesАcould circulate in the oil and grind against the part surfaces causing wear. The oil filter removes many of the particles and sludge, but eventually the oil filter can become clogged, if used for extremely long periods. 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. Д

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, the quickest oil change shops recommended intervals of 5,000 km (3,000 mi)Иor every 3 months which is not necessary according to new car manuals from manufacturers. This has led to a 3,000–mile myth among most Americans.

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; contrary to what most people think, this applies to short trips of under 16 km (10 mi), where the oil does not get to full operating temps long enough to burn off condensation, excess fuel, and other contamination that

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