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
needed to ensure proper cooling of these internal parts.
The oil pump constantly circulates the oil to all vital areas of the
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
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 or grades:
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" in a motor oil's classification 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
Single-grade oils have a limited range of protection and, therefore, a limited number
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
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. SM 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.
Cars from model-years 1980 to 1989 require SF oils, while cars from model-years 1990
to 1993 require SG oils.
New cars beginning with the 1994 model year require oils with an API SH performance rating.
Beginning with model-year 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, CI-4 Plus and CJ-4. All are obsolete except CF, CF-2, CG-4, CH-4, CI-4, CI-4 Plus and CJ-4
performance rated oils.
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 indirectinjected 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.
CG-4 for Heavy-Duty Four-Stroke Diesel Engine Service.
Service category CG-4 is required for severe-duty, high-speed, four-stroke engines
using fuel with less than 0.5% weight sulfur. CG-4 oils are required for engines meeting 1994 emission
standards. Can be used in place of DC, DE and CF-4 oils.
CH-4 for Four-Stroke Diesel Engine Service. CH-4 is required for high-speed, four-stroke engines designed to meet 1998 exhaust
emission standards. CH-4 oils are specifically compounded for use with diesel fuels ranging in sulfur content
up to 0.5% weight. Can be used in place of CD, CE, CF-4 and CG-4 oils.
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
CJ-4 for 2007 and newer diesel Engines. CJ-4 was developed to address special concerns about emission control engines and
their operation on ultra-low sulfur diesel fuel (ULSD). 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.