Motor Oil Functions and Classifications


What is 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 petroleum, one or several synthetic chemicals or 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 rate of "boil off".  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 anit-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 whcih service classification rating and viscosity grade and oil receives.


One of the most important properties of an oil is it's viscosity.  This refers to the internal cohesiveness of the oil or it's resistance to flow.  An oil must be able to flow at low temperatures to lubricate internal moving parts on 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 and oil's viscosity characteristics over a wide temperature range.  The less and 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 a high VI.  The VI is measured by comparing the viscosity of the oil at 40 degrees C with it's viscosity at 100 degrees C.  It does not indicate the performance of the 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 rpm for the engine to start.  The Mini-Rortary Viscometer measures the speed at which a shaft can turn in cold-thickened oil and evaluates the "flowability" of the oil.  An oil must be bale to flow to the oil pump inlet to be circulated through the engine for lubrication and wear protection.  The results of these two simulators are used to assign an oil's SAE "W" grade.  In multi-grade 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.


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 anti-wear 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 the contact.


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 effect of acids, neutralizing additives are blended into the 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 10, which is sufficient for the sulfur content of the diesel fuels found here.  Amsoil manufactures a diesel oil with a TBN of 12.  A typical TBN for a gasoline motor is 5 or 6.

Detergents and Dispersants

Combustion causes carbon build ups 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 unburned and burned 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 deposits suspended in the oil are eventually removed by the oil filter.


Tiny air bubble 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, Slicone, 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% of the engine cooling that takes place.  This cools only the upper portion of the engine, inluding the cylinder heads, walls and valves.

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

Engine heat that 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 dissispatates 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 cooling of these internal parts.  The oil pump constantly circulates oil to all vital areas of your engine.

Classification Systems

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

SAE Grade

The Society of Automotive Engineers viscosity grade system is based on viscosity measures taken from various tests.  This system established eleven distinct motor oil classes or grades: SAE 0W, SAE 5W, SAE 10W, SAE 15W, SAE 20W, SAE 25W, SAE 20, SAE 30, SAE 40, SAE 50 and SAE 60.  These are know as the single grade or single viscosity oils.

These grades correspond to specific ranges that the specific oil falls into. The "W" in the grade indicates that the grade is suitable for use in cold temperatures.  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 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 havea limited number of uses.  In order to increase an oil's usefullness, it must be able to meet requirements of two or more classifications.  Mulit-grade or multi-viscosity oils effectively meet the viscosity requirements of two or more classes.  Examples of multi-viscosity oils are SAE 5W-30, SAE 10W-30 and SAE 20W-50.  The number with a W focuses on the oils properties at low temperatures.  A multi-grade oil liek 10W-30 meets the requirements of SAE class 10W while cold and SAE class 30 when hot.  10W-30 and 5W-30 are widely used because under all but extremely cold or hot conditions, they are light enough for easy engine cranking at low temperatures and heavy enough to protect satisfactorily at high temperatures.

API Class

The American Petroleum Institute 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 classes emphasizes oil properties critical to gasoline or propane fueled engines.  If an oil passes a series of tests in specific engines, the oil can be sold bearing the applicable API service classification.

There are ten S-series classifications: SA, SB, SC, SD, SE, SF, SG, SH, SJ and SL.  The S series classes progress alphabetically as the level of lubricant performance increases.  Each class replaces those before it, with SL currently offering the most protection.  SL 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 us unless specified.  SC oils were required for new car warranties from 1964-1967.  SD oils were required from 1968-1970 and a few in 1971.  Some new car warranties required SE oils fore 1971 and it's use continued until 1979.  New car warranties from 1980-1989 required SF oils.  New cars from 1990-93 require SG oils.

New car warranties beginning with the 1994 model year require oils with an API SH performance rating.  1997 required SJ oils.  The year 2001 brought the introduction of SL oils.  SL oils are designed to increase fuel economy, reduce emissions and protect hot, hard-wroking engines over the course of a very long warranty period.

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

Unlike S-series classes, C-series classes do not supersede one another.  The current classifications, CF, CF-2 and CH-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 in off-road applications, including diesel fuel with greater than .5% 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.

CH-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.  CH-4 oils are especially effective in engines designed to meet 1998 exhaust emission standards.  CH-4 oils may be used in place of CD, CE, CF-4 and CG-4 oils.

The SAE and API classification systmes 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 familair with the SAE system then the API system.

SJ is the cuurent API class.  SJ oils are widely available and most gasoline engine automobiles either specify SJ oil or, if they were manufactured before the SJ class was created, may use SJ oil.  However, one should be sure to purchase the SJ class oil for the best engine protection available.  Of course, motorists should follow the oil specification of their vehicle's 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 10-20% more horsepower.

The smaller engines being turbocharged in some contemporary cars run at higher RPM than normal engines do.  Higher RPM requires 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 due to the higher heat, reducing the cars 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 an the oil before they are shut off.  If the engine is shut off without cooling the turbo bearing and the oil, the oil undergoes "heat soak", along 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 is left 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 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 high 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.

SJ-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 category designation for diesel engines oils begins with the "C" listing, for example, CH-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 of CH-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 with API categories beginning with "S" listings are meant for use in gasoline engines only.  The CF is the SH, CF designation indicates sufficient detergency and oxidation protection for turbo-charged gasoline engines.

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