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What Is a Motor Oil?



Amsoil Oil ChemistMotor oil is the primary determinant in the durability of an engine. It contains two basic components: base stocks and additives.

Base Stocks:

The base stock is the bulk of the oil. The base stock lubricates internal moving parts, removes heat and seals piston rings. 

Motor oil base stocks can be made from: 1) petroleum, 2) chemically synthesized materials, 3) a combination of synthetics and petroleum (called para-synthetic, semisynthetic or synthetic blend.) 

A petroleum base stock consists of many different oil fractions that form the final product. Generally, molecules of a petroleum base stock are long carbon chains that can be sensitive to the stress of heat and “boil off” at relatively low temperatures. Engine temperatures break down these molecular chains, changing the physical properties (such as viscosity) of the motor oil. 

The difference with synthetic base stocks is that molecules are uniformly shaped, which makes them more resistant to the stress of heat. Because AMSOIL synthetic motor oils possess these uniformly-shaped molecules, they have a low “boil off” rate. Thus, their physical properties (such as viscosity) do not change.


The various chemicals that comprise the additive system in motor oils function to provide anti-wear, antifoam, corrosion protection, acid neutralization, maintenance of viscosity, detergency and dispersancy. These are the chemicals that help modern motor oils meet the increasing demands of today’s high-tech engines. Their quality varies widely throughout the lubrication industry, ranging from a bare minimum in some oils (to comply with certain requirements) to exceptionally high quality, as in all AMSOIL motor oils.

What a Motor Oil Must Do

Modern motor oil is a highly specialized product carefully developed by engineers and chemists to perform many essential functions. A motor oil must:

Permit easy starting
Lubricate engine parts and prevent wear
Reduce friction
Protect against rust and corrosion
Keep engine parts clean
Minimize combustion chamber deposits
Cool engine parts
Seal combustion pressures
Be nonfoaming
Aid fuel economy.


Improvements in Oil

The quality of motor oil has changed dramatically in the past 30 years, and new demands on lubricants in modern engine design call for oils that meet stringent requirements. Variations in an oil’s ability to meet the requirements determine which service classification rating and viscosity grade it receives.

Service classifications are determined by the American Petroleum Institute. Viscosity grades of oils are determined by the Society of Automotive Engineers. These two organizations have set industry standards for motor oils for more than 75 years.


Viscosity, the most important property of an oil, refers to the oil’s resistance to flow. The viscosity of oil varies with changes in temperature – thinner when hot, thicker when cold. An oil must be able to flow at cold temperatures to lubricate internal moving parts upon starting the engine. It must also remain viscous or “thick” enough to protect an engine at high operating temperatures. When an oil is used at a variety of temperatures, as it is in most engines, the change of viscosity with temperature variation should be as small as possible.

The measure of an oil’s viscosity change is called the Viscosity Index number (VI); the higher the number, the smaller the viscosity change which means the better the oil protects the engine. The number does not indicate the actual viscosity in high and low temperature extremes of the oil. It represents the rate of viscosity change with temperature change.

Viscosity improvers are viscous chemical compounds called polymers or polymeric compounds that decrease the rate at which oils change viscosity with temperature. These viscosity modifiers extend a motor oil’s operating temperature range and make multi-grade or allseason oils possible. However, lowquality viscosity improvers lend themselves to shearing.

The VI is measured by comparing the viscosity of the oil at 40°C (104°F) with its viscosity at 100°C (212°F). VI can provide insight into an oil’s ability to perform at high and low temperatures.

Petroleum-based motor oils require the use of viscosity improvers to meet the low-temperature requirements of SAE 0W, 5W or 10W and the high-temperature requirements of SAE 30 or heavier oil. Synthetic-based motor oils have a naturally-high viscosity index and require less viscosity improver additive than petroleum oils.

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