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What Is a Motor Oil?
Motor 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,
semi-synthetic 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.
Additives
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 long have set industry
standards for motor oils.
 Viscosity
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 multigrade or
all-season oils possible. 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.
Cold Temperature Protection
Motor oil must begin to circulate as soon as the engine is started. If oil
gets cold enough and begins to solidify, it fails to flow through the oil
screen to the pump at engine start and causes bearings and other critical
parts to fail almost immediately.
Pour Point is an indicator of the
ability of an oil to flow at cold operating temperatures. It is the lowest
temperature at which the fluid will flow. Modern refining techniques remove
most of the wax from petroleum oil, but some wax-like molecules remain.
These wax-like molecules are soluble at ambient temperatures above freezing,
but crystallize into a honeycomb like structure at lower temperatures and
cause oil circulation problems at low temperatures.
Pour Point Depressants
keep wax crystals in the oil microscopically small and prevent them from
joining together to form the honeycomb-like structure. They lower the
temperature at which oil will pour or flow and are found in most motor oils
designed for cold weather use. As synthetic motor oils do not contain those
wax crystals, they do not require pour point depressant additives.
Wear Protection
Since one of an oil’s main functions is to prevent friction and wear,
Anti-wear Additives are part of the chemical composition of an oil. These
additives protect engines by bonding to metal surfaces and forming a
protective film layer between moving parts that are vulnerable to friction
and wear when an engine is first started and before the oil begins to
circulate completely. While this protective film doesn’t entirely eliminate
metal to metal contact of moving parts at start up, it minimizes the effects
of contact. Oxidation
Because excessive engine heat causes chemical breakdown of oil, which in
turn results in permanent thickening of the oil, oxidation inhibitors work
to limit the impact of oxidation. Oil oxidation produces acidic gases and
sludge in the crankcase. These gasses combine with water in the crankcase to
corrode and rust the engine. Corrosion is especially critical in diesel
engines. TBN
An oil’s ability to neutralize acids is expressed by its Total Base Number (TBN).
The greater the number, the greater the amount of acidic byproducts the oil
can neutralize. A high TBN is particularly important in extended-drain
interval oils, such as AMSOIL Motor Oils, because they neutralize acids, and
more of them, for a longer period of time. Most oils for diesel engines in
North America have a TBN between 8 and 12. AMSOIL manufactures several
diesel oils with a TBN of 12.
Detergents
In the same way that some chemical compounds are used to prevent engine rust
and corrosion, other chemicals are added to motor oil to help prevent
combustion by-products from forming harmful sludge or varnish deposits.
Detergents are added to motor oil because combustion causes carbon build-up
and deposit formation on the pistons, rings, valves and cylinder walls.
Carbon and deposits affect engine temperature, oil circulation, engine
performance and fuel efficiency. Detergent additives clean these by-products
from the oil. Some combustion by-products slip past the piston rings and end
up in the motor oil, which can clog the engine’s oil channels.
Dispersants
While detergents help minimize the amount of combustion by-products,
dispersant additives keep those byproducts suspended in a form so fine they
minimize deposits. They keep the oil in the engine clean while they prevent
the build-up of carbon or deposits from burned and unburned fuel and even
from the oil itself. Eventually, these suspended particles are removed by
the oil filter. Anti-Foam
The addition of silicone or other compounds in very small amounts makes most
oils adequately foam-resistant. It’s important to minimize foaming in motor
oil because tiny air bubbles are whipped into motor oil by the action of
many rapidly moving parts, resulting in a mass of oily froth that has very
little ability to lubricate or aid in the cooling of the engine. These
compounds weaken the air bubbles, causing them to collapse almost
immediately upon forming, allowing the oil to continue to protect the
engine. 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, oils should cause seals to expand or “swell” slightly to ensure
continued proper sealing.
Heat Dispersal
Another function of motor oil is to cool the engine. The radiator/antifreeze
system is responsible for about 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 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.
Lubricating an engine actually requires a very small amount of motor oil
compared to the amount need to ensure proper cooling of these internal
parts. The oil pump constantly circulates the oil to all vital areas of the
engine. Classification
Systems
Oil is classified by two systems. One system determines the oil’s viscosity
(the SAE grade), and one determines its performance level, which oil to use
in what type of engine (the API class).
SAE Grade
The Society of Automotive Engineers (SAE) Viscosity Grade is a system
based on viscosity measures taken from a variety of tests. It developed
11 distinct motor oil viscosity classifications 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 single grade or single viscosity oils. These grades
designate the specific ranges that the particular oil falls into. The
“W” indicates the grade is suitable for use in cold temperatures. (Think
of the “W” as meaning “Winter”.) The classifications increase
numerically, readily indicating the difference between them and what the
difference means. Simply put, the lower the number, the lower the
temperature at which the oil can be used for safe and effective
protection. The higher numbers reflect better protection for high heat
and high load situations. SAE 20 and SAE 20W are two separate
classifications. Single grade oils
have a limited range of protection and, therefore, a limited number of
uses. With today’s well-refined, high viscosity index oils, however, an
SAE 20 oil usually will meet the viscosity requirements of SAE 20W and
vice versa. Those that do are classified SAE 20W-20. This multi-grade or
multi-viscosity ability increases an oil’s usefulness, because it meets
the requirements of two or more classifications. Examples of
multi-viscosity oils are SAE 5W-30, SAE 10W-30, SAE 15W-40 and SAE
20W-50.
The number with the “W” designates the
oil’s properties at low temperatures. The other number characterizes
properties at high temperatures. For instance, a multi-viscosity or
multigrade oil such as 10W-30 meets the 10W criteria when cold and the
30 criteria once hot. SAE 10W-30 and SAE 5W-30 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
at high temperatures. 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. The classifications progress alphabetically as the level
of lubricant performance increases. Each classification replaces those
before it.
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 use unless specified. New
cars from 1980 to 1989 require SF oils, while new cars from 1990 to 1993
require SG oils. New cars beginning with the 1994 model year require
oils with an API SH performance rating. Beginning with 1997, new cars
require an API SJ oil. The year 2001 brought the introduction of SL
oils.
SM category is the most recent
classification. It was introduced Nov. 30, 2004. SM oils are designed to
provide improved oxidation resistance, improved deposit protection,
better wear protection and better low temperature performance over the
life of the oil. SJ, SL and SM are the current API classes. SJ, SL and
SM oils are widely available and ensure the best engine protection
available.
C-series classifications pertain
to diesel engines. They are: CA, CB, CC, CD, CD-II, CE, CF, CF-2, CF-4,
CG-4, CH-4, CI-4 and CI-4 Plus. All are obsolete except CF, CF-2, CH-4,
CI-4 and the new CI-4 Plus performance rated oils. However, oils used in
turbocharged gasoline engines retain CF as part of their performance
designation: SM/CF. Not all C-series classifications supersede one
another. The current classifications, CF and CF-2 are specified for
different applications.
CF for Indirect Injected Diesel Engine
Service. Service Category CF denotes service typical of indirect
injected diesel engines and other diesel engines that use a broad range
of diesel fuels in off-road applications, 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. This service category is 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.
CI-4 Plus for Severe Duty Diesel
Engine Service. CI-4 Plus typically is required in high speed
four-stroke diesel engines used in heavy-duty on- and off-highway
applications. CI-4 Plus oils are especially effective in engines
designed to meet 2002 exhaust emission standards. CI-4 Plus oils may be
used in place of CD, CE, CF, CF-4, CG-4, CH-4 and CI-4 oils. These
classification systems aim 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 engine must withstand. |
