Motor Oil Fundamentals

Engine Oil Roles, Modes of Breakdown, & Conventional vs Synthetic Motor Oil

Motor oil is the lifeblood of any engine; without it, an engine would quickly overheat and seize. Engine oil lubricates and cools the mating surfaces between various mechanical components, reducing friction and wear in a highly demanding environment. Lubrication is so important to the automotive sector that there are hundreds of commercially available motor oils on the market, many with claims to provide the best performance and protection for your precious vehicle. This article aims to educate the reader on the fundamentals of engine oil in addition to comparing conventional and synthetic motor oil options.

Crude Oil | The Foundation for Fuels & Conventional Lubricating Oils

Crude oil is the unprocessed petroleum that is conventionally collected from deep beneath the Earth's surface. This thick, black substance is refined into various grades of fuels and oils, including gasoline, diesel fuel, and kerosene, through a process called fractional distillation. Petroleum is comprised of various hydrocarbons and impurities such as oxygen, sulfur, nitrogen, vanadium, and nickel. Through the distillation process, the impurities are removed and the various hydrocarbon products are sorted into categories based on their molecular structure. All forms of petroleum based fuels and oils are nothing more than hydrocarbon compositions of different molecular structure.

Purpose & Role of Motor Oil

The most obvious role of motor oil is to minimize friction between 2 surfaces, reducing wear and heat buildup. In theory, the 2 surfaces never actually come in contact one in another and are instead separated by a thin layer of engine oil, which allows the parts to move against one another with minimal restriction. This is the reason why gear teeth and bearings, amongst other mechanical components, can transfer tremendous forces repeatedly for hundreds of thousands of miles without excessive wear or failure. However, motor oil serves other vital roles that include:

  • Creating a seal between piston rings and the cylinder wall. Although strict manufacturing tolerances allow for a tight fit between the piston rings and the cylinder wall, it is not a perfect seal. The thin layer of motor oil that coats the piston rings and cylinder walls aids in creating a tighter seal, maintaining compression and reducing blowby.
  • Dampening mechanical shock loads, especially those that are repetitive and occur rapidly under high pressure. This includes the force of the connecting rod pushing downwards on crankshaft during a power stroke, a pushrod rapidly acting against a rocker arm to open a valve, and a variety forces created by the complex system of moving parts found inside an internal combustion engine.
  • Acting as a medium to reject heat from vital engine components. Despite motor oil's inherent ability to reduce friction, it still exists between moving parts and a major component of friction is the subsequent heat it creates. Motor oil contacts hot surfaces and draws some of this heat out of the material. Most diesel engines utilize an oil cooler that allows excessive heat to be transfered to the engine cooling system where it can be rejected through the radiator. Motor oil's role as a coolant is especially important in turbocharged engines where it is often used to cool turbocharger bearings.
  • Acting as a medium to remove deposits and contaminants. Particle contaminants (wear metals, deposits, and larger soot particles) are collected by the engine oil filter. Chemical contaminants (fuel, smaller soot particles, and other substances) are often neutralized by additives in the engine oil. Modern engine oils also contain detergents to keep oil passages clean and prevent/remove deposits that may form.
  • Assisting in the function of mechanical components/systems as a hydraulic fluid. In HEUI injection systems, motor oil compressed as high as 3,000-4,000 psi by a dedicated high pressure oil pump is used at each individual fuel injector to create injection pressures of up to 26,000 psi. In a magnitude of vehicles, pressurized motor oil is used via a piston-type actuator to change the position of the vanes in the turbine housing of variable geometry turbochargers. Motor oil, like other liquids, is an incompressible fluid and thus is adequate to use in such hydraulic operations.

Motor Oil Fundamentals

Motor Oil Terminology

API - Acronym for the American Petroleum Institute, an association that, amongst other tasks, develops and publishers standards regarding various aspects of petroleum distillates and products. API standards regarding the classification of engine oils are widely accepted by manufacturers in the United States.

Viscosity - the measurement of a fluid's resistance to flow and the corresponding deformation in liquids. With regard to lubricants, viscosity indicates its thickness. The more viscous a liquid the more it resists flow, i.e. a liquid like honey is considered very viscous whereas water is considered much less viscous; honey has a higher viscosity than water.

Viscosity grade - Classification used to categorize motor oil by its viscosity. 10W, 30W, 10W-30, 15W-40 are all examples of motor oil viscosity grades. Viscosity grades can be singular (straight weights) or multigrade.

Viscosity index - Viscosity index is a unites measurement based on a liquid's change in viscosity relative to a change in temperature. The viscosity index of a motor oil characterizes its viscosity relative to temperature. Although originally developed on a scale of 0 to 100, modern lubricants, particularly synthetic oils, can measure significantly higher. This metric was developed by the Society of Automotive Engineers (SAE) and compares the kinematic viscosity of the oil at 104° F and 212° F.

Pour point - the lowest temperature at which the oil will flow without intervention (i.e. stirring, shaking, etc). Standards developed and published by the American Society for Testing and Materials (ASTM) are generally accepted for measuring the pour point of engine oils.

TBN - Acronym for Total Base Number, a measurement of the basicity of the engine oil. In this context, a base is a solution with a pH level greater the 7. This is opposed to a solution that is acidic, indicated by a pH level less than 7. For reference, water sits at 7, the neutral position on the pH scale (neither a base nor an acid). The TBN of engine oil is particularly important in older vehicles as more blowby is common. Since blowby products (particularly exhaust soot) are acidic, an engine oil with a high TBN will have a greater tendency to neutralize more acidic contaminants in the crankcase than one with a lower TBN.

Understanding Motor Oil Grades & "Weights"

Motor oil is most commonly classified, marketed, and sold by its viscosity grade. A more common term for viscosity grade is "weight". A 10 weight oil, for example, is advertised as 10W, a 20 weight oil as 20W. Despite common misconceptions, the "W" in the oil grade is NOT shorthand for "weight" (more on this later). Engine oil is sold in single grade or multigrade forms, with single grade or "straight weight" oils being more common for small engines and power equipment and multi-grade oils being more customary for automotive engines (with some exceptions). Single grade engines oils (10W, 20W, 30W, etc) adhere to a single viscosity specification, whereas multigrade engine oils (15W-40, 5W-40, 10W-30, etc) must adhere to a dual viscosity specification; one viscosity at a low temperature and a separate at a high temperature.

In single and multigrade engine oils, the "W" denotes "winter", identifying that the number(s) listed before the "W" corresponds to the low temperature viscosity grade and the numbers following the "-" conform to the high temperature viscosity grade (in multigrade oils). The temperatures at which these viscosity measurements are taken depends on the oil grade specified, as outlined by SAE standards. A 15W-40 oil therefore has a viscosity grade of 15 at a low temperature (generally sub-zero on the Fahrenheit scale) and 40 at operating temperature. The numbers themselves are arbitrary and do not directly correspond to any measurable property of the engine oil. Engine oil must adhere to other classification standards as well, but the most significant difference between grades is the viscosity.

Motor Oil Breakdown

Motor oil is subject to mechanical, thermal, and chemical breakdown. The breakdown of oil (or any lubricant for that matter) occurs when the molecular structure or chemical formulation of the lubricant is changed such that it alters the oils chemical properties and/or performance characteristics. Engine oil, whether it be conventional or synthetic, has a finite life and this is the very reason that we drain and replenish the oil in engines frequently. Even the best lubricants eventually breakdown to the point that they no longer adequately perform their role.

Mechanical breakdown, also referred to as shear, is the physical act of breaking apart the molecular structure of the oil through mechanical forces. Since oil acts as a barrier between two surfaces, it is continuously subjected to high pressures and high forces that perpetually break the long hydrocarbon chains into smaller, more volatile, less stable components that no longer offer the same performance characteristics and viscosity of the original formulation.

Chemical breakdown of engine oil is typically due to oxidation and contamination. Oxidation is a chemical reaction that changes the atomic structure of molecules. With regard to lubricants, it typically causes them to thicken and in extreme cases results in the formation of sludge. This viscosity change is widely damaging to the performance characteristics of the lubricating oil. A major contaminant, and likely the most prevalent inside a crankcase, is fuel and exhaust byproducts (particularly in diesel engines, which produce exhaust soot). As previously mentioned, the seal between a piston ring and the cylinder wall and/or piston is not perfect and a small amount of fuel and exhaust gases find there way into the crankcase by means of blowby. Fuel dilution of engine oil is of particular concern in diesel engines because the fuel is heavy and less volatile than fuels such as gasoline, thus it is not easily removed through a crankcase ventilation system. These contaminants not only change the composition of the engine oil, but they are often acidic and react negatively with the oil. Contamination can also come in the form of wear metals. While large metal fragments and particles are removed by the engine oil filter, microscopic particles can fall into suspension with the oil, altering its composition.

Thermal breakdown occurs due to the high operating temperature at which the oil is subjected to inside a running engine. There's two major takeaways regarding thermal breakdown. First and foremost, the reactivity of engine oil increases relative to the temperature it is subjected to (within certain thresholds). This means that hot engine oil is more prone to oxidation and forms of chemical breakdown/contamination than engine oil at room temperature. Oxidation is the same process that causes rust to form on steel; if you heat a steel bar to a red hot glow, rust will begin to form quickly as the reactivity of the metal with the oxygen in the environment is extremely high. However, a cold steel bar will resist rust for a relatively long period of time and it will form slowly. Most elements, compounds, and chemicals react this way - heat causes the reaction to occur more rapidly. Secondly, high heat simply breaks down the molecular chains of the oil over time.

Motor Oil Base Stock Groups

Motor oil begins life as a refined petroleum product or other lubricant known as a base oil, which forms the foundation for the basic chemical composition of a finalized product. These base oils are grouped into 5 categories by the American Petroleum Institute (API); group I to III base oils are refined from crude oil, with group I and II base oils producing the least refined, least expensive, and least desirable lubricants. Although lubricants produced from group I and II base oils produce adequate lubricants in less demanding applications, their not well suited for the high heat and mechanical demands found in internal combustion engines. Group III base oils are heavily refined/processed resulting in the formation of purer and more uniform hydrocarbon structures. Conventional motor oils produced from group III base oils provide favorable performance characteristics but are more expensive than those produced from group I and II base oils. Group III base oils can be used in the production of semi-synthetic and full synthetic lubricants, but the later requires significant processing and chemical engineering.

Group IV base oils are made from polyalphaolefins (PAOs), the most common base oil used in the production of synthetic engine oil and other full synthetic lubricants. However, they do require blending with other base oils and/or additives to overcome their limited disadvantages. This synthetic hydrocarbon remains stable through and is resilient to a broad operating temperature range and can withstand severe heat and pressure. PAOs are chemically engineered such that the base oil features an extremely uniform molecular structure with minimal impurities and no volatile hydrocarbons (those that with a high tendency to vaporize into a gas).

Group V base oils are more-or-less any chemically engineered oils that do not fall into the category of group I - IV base oils, but can also include vegetable oils and other organic products. Group V base oils are often mixed with other base stocks to enhance the properties and performance characteristics of the lubricant. Silicon, esters, and polyglycols are amongst the base oils that fall under the group V category.

API ratings for emissions

The American Petroleum Institute also classifies engine oils with respect to vehicle emissions. This is particularly important on modern diesel vehicles with exhaust aftertreatment systems as a "low ash" oil is required to prevent diesel particulate filters and catalysts from becoming loaded with combusted (or partially combusted) engine oil particles. These particles are not easily removed from filters/catalysts during the regeneration process and thus will eventually plug them. There are two API emissions categories for diesel engines, "C" and "F"; the "F" category is fairly new to the industry. The "C" category has undergone many progressions, with some of the older classifications now retired.

Examples of emissions category include CI-4, which was introduced in 2002 to ensure engine oils were resilient enough in diesel engines equipped with exhaust gas recirculation systems. CJ-4 would later become the standard for engines featuring exhaust aftertreatment systems. FA-4, the only "F" category emissions classification at time of publishing, lays the foundation for higher standards with regard to shear in high temperature applications and promoting lower greenhouse gas emissions. Every manufacturer lists the required engine oil API emissions classification in the owners manual for the vehicle. Note that many older classifications are now defunct and/or non-existent on the market.

Conventional Motor Oil

Conventional motor oil is produced from group I through III base oils, as previously discussed. It is a pure petroleum product save for the additives that the product manufacturer has introduced to the base stock. The difference between a low and high quality conventional motor oil is in the level of refinement and processing of the base stock in addition to the specific additives the product manufacturer has employed to boost the performance and/or longevity of the product.

The lowest quality oils will tend to be developed from group I base oils and contain little to no additives, resulting in a product that is highly sensitive to high temperature and high pressure applications. Note that most motor oils are not produced from group I base oil since it lacks the inherent properties needed to protect an engine. A higher quality conventional oil will start life as a group III base oil and be processed such that the final product has a very uniform molecular structure. However, one of the major issues with conventional motor oil is in achieving this uniform composition, which is where synthetic motor oil comes into the picture.

Synthetic Motor Oil

Synthetic motor oil is chemically engineered such that the molecular structure of the final product is uniform and selected based on the performance requirements of the application. The term "synthetic" is introduced because these compounds are synthesized, or man-made. Most synthetic motor oils fall into the group IV and V base oil category, although synthetic oils can be produced from group III base oils that have been chemically modified. It is a combination of this high grade base oil in addition to manufacturer specific additives that give many advantages to the chemical properties and performance characteristics of synthetic oils.

Advantages of Synthetic Motor Oil

Synthetic motor oils are rapidly becoming the industry standard for modern vehicles, with conventional oils slowly being phased out in many applications due to the overwhelming benefits that synthetic lubricants provide. Synthetic lubricants generally offer:

  • Superior protection against engine wear
  • Superior performance stability across a broad operating temperature range, including extremely low and high temperature conditions
  • Superior chemical stability and resilience to chemical breakdown
  • Superior resilience to shear and thermal breakdown - particularly important in turbocharged engines where engine oil is used to cool the bearings in a turbocharger
  • Greater resilience against oxidation, deposit formation, and sludging
  • Lower coefficient of drag, improved flow in sub-zero temperatures
  • Increased fuel efficiency and lower greenhouse gas emissions
  • Reduced buildup in oil passages and on critical components
  • Greater protection in vehicles that are stored for extended periods of time or driven infrequently

Synthetic motor oils are completely cross-compatible with conventional motor oil and you can start using synthetic fluids at any time. Synthetic lubricants provide the same benefits in brand new and high mileage vehicles, and there is no flushing or cleaning process necessary to convert to a synthetic oil. In addition to the aforementioned advantages of using synthetic lubricants, we've noted the following benefits specific to diesel engines observed while using Amsoil synthetic diesel oils:

  • Reduced oil consumption in high mileage engines
  • Prolonged turbocharger bearing life
  • Reduced stiction related problems and prolonged fuel injector life in HEUI applications (7.3L and 6.0L Power Stroke engines)
  • Reduced fuel dilution in vehicles equipped with diesel particulate filters
Disadvantages of Synthetic Motor Oil

The single disadvantage is the purchase price, which can be considerably higher than that of top-shelf conventional motor oils. Synthetic lubricants are slowly edging out conventional oils due to their long list of advantages and the inability of petroleum lubricants to meet the intense demands of modern engines. Note that the average price of synthetic motor oil has decreased in recent years due to the growing prevalence of synthetic options on the market.

Semi-Synthetic & Synthetic Blend Motor Oil

Semi-synthetic and synthetic blends of motor oil are a cost effective upgrade that offers some of the benefits that synthetic lubricants provide but at a lower price. These mixtures of conventional and synthetic oils are, in our opinion, obsolete and no longer an attractive option due to the current availability and pricing of synthetic motor oils.

Oil Recommendations - What's the Best Oil for my Engine?

One of the more prevalent and leading manufacturers of high quality synthetic lubricants is Amsoil Inc. Amsoil produced the first synthetic motor oil in the early 1970's that met American standards as outlined by the API. In our opinion, the quality of their products is unmatched and there's no better motor oil for your vehicle. Amsoil's line of full synthetic lubricants includes motor oil for diesel engines, gasoline engines, 4 stroke power equipment, ATVs, UTVs, construction/agricultural equipment, personal watercraft, and marine applications. We urge you to learn more about Amsoil products by visiting Amsoil - The First in Synthetics®.

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Disclaimer - is an independent Amsoil dealer; contributions to this article were made by an authorized independent Amsoil dealer. This article is not sponsored by nor endorsed by any manufacturer mentioned herein.

Sources - Amsoil Inc, American Petroleum Institute, SAE International