Motor Oil Functions & Classifications

(Note: The following document is from a 1996 printing. Since that time there have been several upgrades to the "S" & "C" classification systems. The basic information however remains the same. We hope to have an upgraded version available on line in the next several months.)

What Is a Motor Oil?

The four primary purposes of a motor oil are lubrication of all moving internal engine parts to minimize friction and wear, cooling the engine, contamination and corrosion control, and sealing the piston rings and other mating surfaces.

A motor oil is made up of two basic components: base stock and additives. The base stock makes up the majority of the completed oil. The functions of the base stock include lubrication of internal moving parts. heat removal and sealing piston rings. Motor oil base stocks can be made from:

1. Petroleum
2. One or Several Synthetic Chemicals
3. A Combination of Synthetics and Petroleum (Called Para-synthetic, Partial Synthetic or Synthetic-Fortified).

A petroleum base stock consists of many different oil fractions blended together to form the final product. Some of these fractions will "boil off" at relatively low temperatures. Generally. molecules of a petroleum base stock are long carbon chains which are sensitive to the stress of heat. Engine temperatures encourage these chains to break down. changing the physical properties (such as viscosity) of the motor oil.

A synthetic base stock is generally composed of uniformly shaped molecules that are resistant to the stress of heat. AMSOIL Synthetic Motor Oils exhibit a very low "boil off" rate. There are no excessively long carbon chains in their structure to break down and alter the physical properties (such as viscosity) of the oil.

The functions of the additive system include anti-wear, anti-foam, corrosion protection, acid neutralization, maintenance of viscosity, detergency and dispersancy. The additive system is composed of various chemicals that do these specific jobs. Their quality varies widely throughout the lubrication industry, ranging from a bare minimum in some oils (to just meet certain requirements) to exceptionally high quality, as in all AMSOIL Motor Oils.

Improvements in Oil

Motor oils today are vastly different from motor oils 30 years ago. Because of new demands placed on lubricants due to modern engine design, today's oils must meet stringent requirements in areas of viscosity, wear protection, corrosion protection, acid neutralization, detergency and dispersancy. Variations in these factors determine which service classification rating and viscosity grade an oil receives.

Viscosity

One of the most important properties of an oil is its Viscosity. This refers to the internal cohesiveness of the oil or its resistance to flow. An oil must be able to flow at low temperatures to lubricate internal moving parts upon starting an engine. An oil must also remain viscous or "thick" enough to protect an engine at high operating temperatures.

Viscosity Index Improvers are additives that extend a multi-grade motor oil's viscosity range. Viscosity Index (VI) indicates an oil's viscosity characteristics over a wide temperature range. The less an oil's viscosity changes with temperature changes, the better the oil protects the engine. An oil that changes little with temperature changes receives a high VI and an oil that changes greatly with temperature changes receives a low VI, Better oils have high VIs.

The VI is measured by comparing the viscosity of the oil at 40"C (104"F) with its viscosity at 100"C (212"F), It does not indicate the performance of an oil at low or high temperatures.

Cold Temperature Protection

Pour Point refers to the temperature at which an oil has solidified, due to cold temperatures, and can no longer pour. The Cold Crank Simulator measures the speed at which a shaft can turn in cold-thickened oil and evaluates the "startability" of the oil. An engine must be able to reach a turning speed above 300 rpms for the engine to start. The Mini-Rotary Viscometer measures the speed at which a shaft can turn in cold-thickened oil and evaluates the "flowability" of the oil. An oil must flow to the oil pump inlet to be circulated through the engine for lubrication and wear protection. The results of the Cold Crank Simulator and Mini-Rotary Viscometer are used to assign an oil's SAE "W' grade. In multigrade or multi-viscosity oils such as 10W-30 and 10W-40, chemical additives called Pour Point Depressants are often added to the oil to allow it to flow enough at cold temperatures for the oil pump to be able to circulate the oil to all parts of the engine.

Volatility

Petroleum multi-viscosity oils have various chemical ingredients in them that allow them to protect engines in a wide range of temperatures. Some of these ingredients are used to keep the oil flowing when it is cold. However, these ingredients are very volatile, and through their normal use in an engine (especially when it is hot), these ingredients evaporate, or boil off. Lighter fractions of a petroleum base stock also boil off in heat. which changes the viscosity of the remaining oil. The oil left behind in the crankcase is thicker and doesn't flow easily when it's cold. This thick oil can take up to five minutes to freely circulate in a cold engine after starting it. Meanwhile the engine is being starved of oil and not being protected against friction and wear.

Wear Protection

One of an oil's main functions is to lubricate moving parts to prevent friction and wear. The oil forms a thin film on, around and between parts which work in close proximity with one another. Ideally, a constant flow of oil keeps those parts from rubbing together.

Metal to metal contact creates tremendous frictional and thermal forces that can actually weld moving parts together, causing the engine to seize. Today's modern oils do a very good job of preventing engine seizure once the engine has warmed up. At start-up, however, when there is little oil on moving parts to lubricate them, metal to metal contact can occur. A good motor oil contains Antiwear Additives to protect engines by bonding to metal surfaces and forming a protective layer between moving parts. This layer does not prevent their rubbing together but minimizes the effects of contact.

Oxidation

Oxidation is the chemical breakdown of oil due to the extreme heat in the engine. Oil oxidation can cause acidic gasses and sludge to form in the crankcase. Acidic gasses combine with water in the crankcase to corrode and rust the engine. Corrosion is especially critical in diesel engines.

To counteract the effects of acids, neutralizing additives are blended into motor oil. An oil's neutralizing capability is expressed by its Total Base Number (TBN). Most oils for diesel engines in North America have a TBN between 7 and 14, which is sufficient for the sulfur content of diesel fuels found here. AMSOIL manufactures a diesel oil with a TBN of 12. A typical TBN for a gasoline engine motor oil is 5 or 6.

Detergents and Dispersants

Combustion causes carbon buildup and deposit formation on the pistons, rings, valves and cylinder walls. Carbon and deposits affect engine temperature, oil circulation, engine performance and fuel efficiency. Additionally, some combustion by-products slip past the piston rings and end up in the motor oil. These by-products can clog the engine's oil channels.

Detergents are added to the oil to keep the engine clean. They prevent the build-up of carbon or deposits from burned and unburned fuel and even from the oil itself.

Dispersants are added to prevent the agglomeration of sludge and dirt in the oil. Dispersants hold the dirt particles in suspension rather than allowing them to come together and form deposits. The particles suspended in the oil are eventually removed by the oil filter.

Foaming

Tiny air bubbles are constantly being "whipped" into motor oil by the action of many rapidly moving parts inside the engine. The result is a mass of oily froth called Foam. This foam has very little capacity to lubricate or aid in the cooling of the engine. It is important to minimize foaming in motor oil. The addition of silicone or other compounds in very small amounts makes most oils adequately foam-resistant.

Seal Swell

All motor oils must be compatible with the various seal materials used in engines. Oil must not cause seals to shrink, crack, degrade or dissolve. Ideally, oil should cause seals to expand or "swell" slightly to ensure continued proper sealing.

All AMSOIL Motor Oils exhibit excellent seal compatibility with Fluorocarbons, Buna-N, Silicone, Viton and other commonly used seal materials.

Heat Dispersal

Motor oil helps cool the engine. The radiator/anti-freeze system is responsible for only 60 percent of the engine cooling that takes place. This cools only the upper portion of the engine, including the cylinder heads, cylinder walls and valves.

The other 40 percent is cooled by the oil. The oil is directed onto hot surfaces, such as the crankshaft, main and connecting rod bearings, the camshaft and its bearings, the timing gears, the pistons and many other components in the lower portion of the engine that directly depend on the motor oil for cooling.

Engine heat is created from both friction of moving parts and the ignition of fuel inside the cylinder. Oil carries heat away from these hot surfaces as it flows downward and dissipates heat to the surrounding air when it reaches the crankcase.

The amount of oil required to lubricate an engine is actually very small when compared to the amount needed to ensure proper cooling of these internal parts.

The oil pump constantly circulates the oil to all vital areas of your engine.

Classification Systems

Oil is classified according to three systems. One system determines the oil's viscosity (the SAE grade), and two systems determine its performance level, i.e. which oil to use in what type of engine (the API class and the ILSAC class).

I. SAE Grade

The Society of Automotive Engineers (SAE) Viscosity Grade is a system based on viscosity measures taken from a variety of tests. This system established eleven distinct motor oil viscosity classifications or grades: SAE0W, SAE5W, SAE10W, SAE15W, SAE20W, SAE25W, SAE20, SAE30, SAE40, SAE50 and SAE60. These are known as single grade or single viscosity oils.

These grades correspond to specific ranges that the particular oil falls into. The "W" in the classification indicates that the grade is suitable for use in cold temperatures. (You can think of the "W" as standing for "Winter.") The classifications increase numerically, allowing you to tell the difference between them and what this difference means. In simple terms, the lower the number, the lower the temperature at which the oil can be used for safe and effective protection. The higher the number, the better protection offered for high heat and high load situations.

Single grade oils have a limited range of protection and so have a limited number of uses. In order to increase an oil's usefulness, it must be able to meet the requirements of two or more classifications. Multigrade or multi-viscosity oils effectively meet the viscosity requirements of two or more classifications. Examples of multi-viscosity oils are SAE5W-30, SAE 10W-30, SAE 15W-40 and SAE20W-50. The number with a "W" focuses on an oil's properties at low temperatures. The number without a "W" characterizes properties at high temperatures. A multi-viscosity or multi-grade oil, e.g. 10W-30, meets the 10W criteria when cold and the 30 criteria once hot. SAE10W-30 and SAE10W-40 are widely used because under all but extremely hot or cold conditions, they are light enough for easy engine cranking at low temperatures, and heavy enough to protect satisfactorily at high temperatures.

Choose your winter oil according to the lowest temperature your region experiences and your summer oil according to the highest temperature your region experiences.

(SLS Note: This recommendation is the old standard based on the use of oils manufactured using petroleum base stocks. Due to the much wider operating range of AMSOIL synthetics, there is no need to run different viscosities for different seasons.)

II. API Class

The American Petroleum Institute (API) developed a classification system to identify oils formulated to meet the operating requirements of various engines. The API system has two general categories: S-series and C-series.

The S-series service classification emphasizes oil properties critical to gasoline or propane fueled engines. If an oil passes a series of tests in specific engines (API Sequence tests), the oil can be sold bearing the applicable API service classification.

There are eight S-series classifications: SA, SB, SC, SD, SE, SF, SG, and SH. The S-series classifications progress alphabetically as the level of lubricant performance increases. Each classification replaces those before it, with SH currently offering the most protection. SH oil may be used in any engine, unless the engine manufacturer specifies a "non-detergent" oil.

SA and SB are non-detergent oils and are not recommended for use unless specified. SC oils were required for new car warranties from 1964 to 1967. SD oils were required from 1968 to 1970 and a few in 1971. Some new car warranties required SE oils in 1971 and its use continued through 1979. New car warranties from 1980 to 1989 require SF oils. New car warranties from 1990 to 1993 require SG oils.

New car warranties beginning with the 1994 model year require oils with an API SH performance rating. SH oils are designed to increase fuel economy, reduce emissions and protect hot, hardworking engines over the course of a very long warranty period.

(SLS Note: Since this document was published, the SJ & SL classifications were finalized and are now the current standard. There is a list at the end of this document that provides the Catagory list for Gasoline, Diesel and Two-Cycle oils.)

C-series classifications deal with diesel engines. There are eight classifications: CA, CB, CC, CD, CD-II, CE, CF-4, CG-4 and CF-4. CA, CB, CC and CD performance rated oils are no longer used in diesel engines. However, oils used in turbocharged gasoline engines retain CD as part of their performance designation: SH, CD.

Unlike S-series classifications, C-series classifications do not supersede one another. The current classifications, CF, CF-2 and CG-4, are specified for various applications.

CF for Indirect Injected Diesel Engine Service. Service Category CF denotes service typical of indirect injected diesel engines and other diesel engines which use a broad range of diesel fuels, including diesel fuel with greater than 0.5 percent sulfur by weight. CF oils may be used in place of CD oils.

CF-2 for Two-Stroke Diesel Engine Service. Service Category CF-2 denotes service typical of two-stroke engines requiring highly effective control over cylinder and ring-face scuffing and deposits. CF-2 oils may be used in engines for which CD-II oils are recommended.

CG-4 for Severe Duty Diesel Engine Service. Service Category CG-4 denotes service typical of high speed four-stroke diesel engines used in heavy-duty on- and off-highway applications. CG-4 oils are especially effective in engines designed to meet 1994 exhaust emission standards. CG-4 oils may be used in place of CD, CE and CF-4 oils.

(Note: CH-4 oils have replaced the CG-4 oils and a replacement for the CH-4 oils (CI-4) should be available shortly.)

The SAE and API classification systems are intended to help motorists choose the right oil for their needs. The choice depends on the engine, the outdoor temperature and the type of driving the motorist does most. Most motorists are more familiar with the SAE Viscosity Grade system than they are with the API Class system.

SL is the current API class. SL oils are widely available and most gasoline engine automobiles either specify SL oil or, if they were manufactured before the SL class was created, may use SL oil. However, one should be sure to purchase SL class oil for the best engine protection available. Of course, motorists should follow the oil (viscosity) specification of their vehicle owner's manual.

Turbocharged Engines

Turbocharged engines (both gasoline and diesel) are different from conventional combustion engines and thus have different lubrication requirements.

A turbocharger compresses incoming air, then feeds this compressed air into the intake manifold, packing a higher density air to fuel mixture, which upon combustion can yield as much as 10 to 20 percent more horsepower.

The smaller engines being turbocharged in some contemporary cars run at higher RPMs than normal engines do. Higher RPMs require a higher quality oil because more stress is placed on the engine.

Turbo engines run much hotter than normal engines run. Heat causes three main problems for lubricating oil. First, wear-promoting viscosity losses are their most damaging in heat. Second, there is more carbon build-up and deposit formation with the higher heat, reducing the car's performance and efficiency. Third, oxidation occurs more rapidly at higher temperatures, which increases corrosion.

Because turbocharged engines run so hot, they should be idled to cool the turbo bearing and the oil before the engine is shut off. If the engine is shut off without cooling the turbo bearing and oil, the oil undergoes heat soak, a long period of exposure to very high heat. Heat soak is extremely harmful to oil. The more volatile portions of the oil boil off and what oil is left oxidizes to form a crusty layer inside the turbocharger. This crust will eventually flake off, wedge between close-fitting components in the turbocharger, and grind and gouge surfaces, altering clearances and generating more abrasive debris. Eventually, this condition leads to the total destruction of the turbocharger, requiring costly repair or replacement.

To combat these problems, a turbo oil of API service classification "CF" is required. API CF turbo oils should have high quality VI Improvers, so there won't be a breakdown in viscosity performance. CF oils specify higher detergent levels to help keep the engine free of carbon build-up and deposit formation. In addition, increased levels of anti-oxidants protect the engine from oxidation and corrosion problems.

SH-CF oils are appropriate for use in turbocharged gasoline engines. They are not appropriate for use in diesel engines, though the "CF" may make it appear so.

The API Service Category designation for diesel engine oils begins with the "C" listing, for example, CG-4, SH. Because fleets often simplify maintenance practices by allowing use of one oil for all fleet vehicles, diesel engine oils are sometimes formulated for use in gasoline engines. Such oils earn a designation such as CG-4, SH. Such oils provide adequate performance in gasoline engines though they are primarily formulated for use in heavy-duty diesels.

On the other hand, most gasoline engine oils are used in non-fleet vehicles so they are not required to pull "double duty" as both gasoline and diesel engine lubricants. They are not formulated with the special needs of heavy-duty diesel engines in mind. Oils whose API Service Category begins with an "S" listing are meant for use in gasoline engines only. The "CF" in the SH, CF designation indicates sufficient detergency and oxidation protection for turbo-charged gasoline engines.

III. ILSAC Class

The 'Starburst' Certification Mark indicates an oil has met the current Minimum Performance Standard for Passenger Car Engine Oils issued by the International Lubricant Standardization and Approval Committee (ILSAC). ILSAC standards are a cooperative effort of the American Automobile Manufacturers Association (AAMA) and the Japan Automobile Manufacturers Association, Inc. (JAMA). The ILSAC GF-2 standard replaced the previous GF-1 requirements on August 1, 1997. The GF-2 standard corresponds to the API SJ category Work is now under way on new and revised tests for the next standard -- GF-3.

Always follow recommended SAE, API and ILSAC standards in your engine owner's manual to be assured of proper protection for your engine.

Engine Oil Service Category Chart (as of 2/28/02)

Gasoline Engines

Note: API Intentionally Omitted "SI" From The Sequence Of Categories Because The Letters Are Commonly Used To Refer To International Units Of Measurement

Diesel Engines

Two Cycle Engines

The present API "TC" rating corolates closely to the NMMA TC-W3 Rating

In 1993, JASO defines a specification that describes three quality levels: FA, FB and FC. (Detergency, Lubricity, Exhaust Blocking, and Smoke) The low smoke requirement is especially stringent for the FC performance category.

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