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Why do you have to change your motor oil on a regular basis?
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Motor oil breaks down over time. When it breaks down, it loses its
effectiveness and can no longer properly protect your engine.
In addition to lubricating an engine's moving parts, motor oil is designed to
carry combustion by-products away from the pistons and cylinders. It is
designed to deal with the small amounts of water that form as the engine heats
and cools, and to collect the dirt and dust that enter the engine through the
air-intake system. It also handles acids that are formed by the reaction
between water and other contaminants. Sometimes there are even fuel leaks
(fuel dilution) or coolant leaks that get into the oil system.
As a car is driven, the level of contamination in the motor oil constantly
increases. The oil filter removes particles as the oil passes through the
filter, but over time an oil's additives are used up and the oil itself can
start to degrade (oxidize or thicken). At that point, the oil can no longer do
its job and must be changed.
The rate at which contamination and additive depletion occurs depends on many
variables. One of these is driving conditions, which vary greatly and have a
direct effect on the useful life of the oil. Other factors include the
precision of ignition, fuel injection or carburetion adjustments, air cleaner
service and the general mechanical condition of the engine.
Oil should be changed before the contamination level reaches the point where
engine damage can result. Because it is difficult for the individual motorist
to determine when the contamination level is too high, automobile
manufacturers provide recommended oil change intervals. These change
recommendations vary by model year and manufacturer. Recommended intervals and
mileage limits also vary with the type of service under which a car operates.
More frequent oil changes are recommended for severe service.
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Is a "straight weight" oil synonymous with a non-detergent oil?
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Not necessarily. "Straight weight" refers to an oil's viscosity, and usually
indicates an absence of viscosity index improvers. "Non-detergent" refers to
the absence of detergent additives that are used to handle combustion
by-products.
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What role does viscosity play in a motor oil?
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Engineers work to establish an optimal viscosity for an oil, based on load and
speed conditions. They balance lighter – or low-viscosity – oil, which
provides little resistance to motion thereby saving fuel and efficiently
transferring horsepower, with a heavier – or high-viscosity – oil that resists
being squeezed out of the contact area between metal surfaces.
The complicating factor is that the viscosity of an oil varies with changes in
temperature – thinner when hot, thicker when cold. At low temperatures, we
need the motor oil to flow readily (not thicken too much or gel). At high
temperatures, we need the motor oil to keep from becoming too thin and
allowing metal-to-metal contact. Therefore, engineers developed multigrade
motor oils.
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What does viscosity mean?
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Viscosity is a measure of a fluid's resistance to flow. A fluid with low
viscosity flows easily and is often called "thin." Water is an example of a
fluid with a relatively low viscosity. A fluid with high viscosity is often
described as "thick." Maple syrup is an example of a fluid with a relatively
high viscosity.
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My car emits smoke when I start up. What does the smoke indicate?
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If the cloud is relatively blue or blue/black, it may indicate that oil is
being burned along with the fuel. The possible cause may be either worn piston
rings or an oil viscosity that is too low.
If the cloud is black, it indicates that excess fuel is being burned.
If the cloud is white, it may simply be the moisture in the cold engine and
exhaust system being burned off. If there's a lot of white smoke and it
continues for a long time, you may have an internal coolant leak.
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What is the API service category classification system?
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Engine oils are currently classified by a two-letter code. Gasoline engine oil
categories start with the letter S (originally designated "Spark Ignition"
engine oils, we now associate the S with "Service"). Diesel engine oil
categories start with the letter C (originally designated "Compression
Ignition" engine oils, we now associate the C with "Commercial").
The second letter is simply a sequential designation of improving quality
levels over time. In other words, when a new industry quality level is
established, the next letter of the alphabet is used (so SJ replaces SH). The
letters "I" and "K" were purposefully skipped to eliminate potential confusion
with other commonly used designations.
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What does a viscosity index number mean?
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The viscosity index (VI) number is a measure of the relative change in
viscosity of oil over a temperature range. The HIGHER the viscosity index, the
SMALLER the viscosity change over temperature. The VI is not related to the
actual viscosity or SAE viscosity, but is a measure of the rate of viscosity
change.
The VI number is typically used only as an indicator. The actual performance
results of low-temperature pumpability tests and high-temperature wear tests
of a motor oil are better predictors of good performance in an engine.
Generally, multigrade oils (0W-40, 10W-30, etc.) will have high viscosity
indexes. Monograde oils (SAE 30, 40, etc.) will have lower viscosity indexes.
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What causes my vehicle's oil warning light to go on?
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Your oil warning light can come on for a number of reasons including: Low oil
level, a failing oil pump, a faulty oil-pressure sensor, blockage in the oil
system, excessive foaming of the oil and more. In all cases, you should shut
down your engine as quickly as it is safe to do so. Continuing to operate your
engine with low oil pressure can result in serious engine damage.
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