Oil Life Monitor -- How Does It Know?
Oil Life Monitor --
How Does It Know?
How long will oil last in an engine? What
reduces the oilís effectiveness? When should
it be changed?
Lubrication engineers perform a number
of tests to answer these kinds of questions.
Vehicles are operated under prescribed conditions,
and periodically a sample of the oil is
taken into the laboratory for analysis. When
the condition of the oil is no longer satisfactory,
the mileage is noted.
From controlled testing like this, engineers
in the past have determined two sets
of mileage numbers, one number for normal
driving and the other for severe conditions.
Severe conditions can mean that the vehicle
is driven hot (for example, pulling a trailer up
a mountain) or is driven such that the oil
never warms completely (for example, trips
less than 5 or 10 miles in a winter climate). It
is then up to the owner to decide whether
their own driving is normal or severe and to
change the oil accordingly.
Now, science and technology have found
a way of taking the guesswork out of the picture.
GM is installing an oil life monitor in an
increasing number of new vehicles. Using a
simple indicator lamp or readout on the
instrument panel, this system notifies the driver
when to change the oil.
Straight oil is not an ideal lubricant in an
engine. A package of additives is needed to
give the oil properties it does not naturally
have or to enhance its natural properties.
Some of the tasks accomplished by additives:
- viscosity modifiers, to keep the oil the
proper thickness over a wide range of
- anti-oxidant, to keep the oil from
- corrosion inhibitors, to protect
- detergents, to suspend solid particles.
What Makes Oil "Wear Out?"
Water in oil, resulting from extreme short-trip
driving, photographed through a transparent
If you were to start out with a crankcase
full of fresh, clean oil, and drove the vehicle
for a period of time, eventually the oil would
have to be changed. During this time, what
can change fresh oil into "worn out" oil?
First, dilution. When gasoline is burned in
the combustion chamber, the by-products
include a lot of water. Some of this water can
find its way into the crankcase through piston
ring blow-by. If the engine is cold, and if combustion
is not perfectly complete, a small
amount of acid is formed. It, too, can blow-by
into the oil. You donít need to be a top-notch
scientist to realize that water and acid arenít
good things to pump through the lubrication
system of the engine. If an engine is run long
enough for the engine oil to warm, the water
and acids will evaporate and not accumulate.
But, during very short trips in cold weather,
water and acids can enter the engine oil and
cause the oil to "wear out."
Second, the degradation of the oil and its
additives. We mentioned earlier that a number
of additives are put into oil to improve its
performance. If these additives are degraded
or decomposed, the oil is no longer capable
of doing all of its jobs properly. Oil with
degraded additives can become thick and
dark. Additives become degraded by exposure
to extreme heat. There are two places a
lot of heat can reach the oil. One is near the
combustion chamber. Oil at the top piston
ring is exposed to very high temperature.
And some bearing surfaces can also put a lot
of heat into the oil at high operating temperatures.
So, degradation of additives from high
temperature operation is the second factor
that can cause oil to "wear out."
How Can Operating
Conditions be Used to Predict
Using carefully controlled laboratory
tests, itís possible for lubrication engineers to
measure how long it takes to dilute engine oil
during cold operation. And itís possible to
measure how long it takes for high temperature
to degrade the additives.
We usually think of measuring time in
hours and minutes, but for an engine, the
amount of revolutions it has run is also a
good measure. So for the purposes of oil life,
time is measured in engine revolutions.
Engineers like to talk in terms of models.
A model is a way to describe something
mathematically. Itís possible to create an oil
life model that very carefully matches the
results of analyzing the oil in a laboratory.
The oil life monitor, then, is based on a
model. A computer chip in the Powertrain
Control Module is loaded with a certain number
of engine revolution counts. The count
for each engine/vehicle combination is determined
by testing. As the engine runs, each
revolution is subtracted from the remaining
count in the oil life monitor. When the count
reaches zero, the instrument panel light
comes on. But, hereís the clever part. When
the various input sensors detect that the
engine is running under either cold or hot
conditions, it subtracts extra counts (penalties)
for each engine revolution. So, the conditions
that cause the oil to "wear out" make
the counter run down faster.
When the oil is changed, itís necessary to
reset the oil life monitor and the
countdown begins again.
NOTE: Synthetic oil resists "wearing out"
better than mineral oil, so the oil life monitor
is set to account for this, but only on vehicles
that are specified for synthetic oil from the
factory -- the Corvette, for instance. Using
synthetic oil in other vehicles is certainly not
harmful, but the oil life monitor will continue
to count down as though the engine contained
Special thanks to TeckLink 3/2002
Special thanks to - Shirley Schwartz
Re: Oil Life Monitor -- How Does It Know?
GM Oil Life System Revisited
Briefly, engine oil degrades in a
predictable fashion, according to several
measurable engine operating conditions.
The engine control module counts
combustion events (measured in rpm)
and reads coolant temperature. From
these numbers, the computer is able to
track oil deterioration and notifies the
driver when a change is needed.
The best value from the cost of an oil
change is obtained by maximizing the
mileage between changes, so long as
there is no adverse effect to the engine.
With the GM oil life system, the average
person can expect oil change intervals of
4000-7000 miles for mixed driving, and
7000 to 10000 miles for highway driving,
while the Chevrolet Corvette and the
2002 Envoy, Bravada and TrailBlazer can
achieve 15000 miles under ideal
Since the GM oil life system first
appeared on some 1988 Oldsmobiles,
over 10 million have been built,
presently at the rate of over 3 million a
year. By model year 2003,
GM expects to install oil life
systems on essentially all
cars and light duty trucks.
This past May, the GM
oil life system development
team was honored by
receiving the first ever
in Transportation award
from the Society of
Automotive Engineers. This
award recognizes that the
extended range offered by
the oil life system can save
huge amounts of new oil,
and can keep thousands of
gallons of used oil out of
There’s a lot of information on vehicle
maintenance shared on consumeroriented
websites – some correct, come
erroneous, and some simply outdated.
For instance, conventional wisdom calls
for oil changes every 3000 miles. Not
surprisingily, this conservative figure is
also supported by those who derive
income from selling oil changes. Many
of your customers have become
convinced that any longer oil change
interval is somehow harmful to their
At the retail level, you can do your
part by promoting proper use of the GM
oil life system. Become familiar with its
function, and be prepared to help
customers understand that observing
the montor’s recommendation is the
easiest way to take the guesswork out
of oil change intervals. It also ensures
that they are giving their vehicle the
proper care it deserves, at the minimum
Special Thanks to David Staley and Chuck Burns
Special thanks to Techlink ---- 11/2001
Re: Oil Life Monitor -- How Does It Know?
GM Oil Life System Revisited #2
How often should engine oil be changed?
- 3,000 miles
- 5,000 miles
- 7,500 miles
- 10,000 miles
Actually, all of these are correct, depending on operating conditions. Oil life is affected by many factors other than just miles driven. The type of driving, temperature, and engine load all play a part.
Thatís why GM has developed the GM Oil Life System, an electronic watchdog that keeps track of all these variables and notifies the driver when itís time to change oil. We first told you about the GM Oil Life System in the March 2000 TechLink. Since then, the system has become standard equipment on nearly all GM products.
Briefly, the Oil Life System is programmed with a certain number of engine revolutions. As the engine runs, this number is reduced until it reaches zero, and the Oil Life light or message comes on. But thereís more. Operating the engine under low or high temperatures, and under high load conditions subtracts (penalizes) extra revolutions, so the light comes on sooner.
Changing engine oil according to actual need rather than an inflexible schedule provides several benefits.
First is simpified determination about when to change oil. No more decisions about ďnormalĒ conditions vs. ďsevereĒ conditions. Second is reduced operating costs for GMís customers, who now have to change oil only when itís needed. Third is minimizing the amount of used oil that must be disposed of. And fourth, engines will always be running with sufficiently fresh oil, for long life.
These benefits will be realized only if engine oil is actually changed as indicated by the GM Oil Life System.
Some customers ďget itĒ when itís explained to them. Others may be reluctant to deviate from traditional oil change interval charts. So, part of the responsibility falls on retail service people to help get the message out.
Traditionally, the vehicle maintenance schedule has been based on miles or time, while the oil change interval is now based on the GM Oil Life System. This could result in customers having to bring their vehicles in for an oil change when the light comes on, only to find that the vehicle is due in a month for scheduled maintenance.
Thatís all changing. In the accompanying article ďSimplified Maintenance SchedulesĒ, youíll learn how maintenance intervals are now being tied into the oil change intervals indicated by the Oil Life System.
Sepcial Thanks to Jerry Garfield and Chuck Burns
Special Thank to TechLink 5/2003
Re: Oil Life Monitor -- How Does It Know?
GM Oil Life System, One More Time #3
Where do you stand on the following issues?
1. Do you want to continue to build customer loyalty and grow your business?
2. Do you want to show your customers you truly have their best interests at heart?
3. Do you want to help improve the environment?
4. Would you like to quit being a part of the throw-away generation?
GM’s development of the Oil Life System and its relationship to the new Simplified Maintenance Schedule have a direct bearing on how well you can answer YES to all of these questions.
How the Oil Life System Works
We’ve explained the GM Oil Life System (GMOLS) in detail before (March 2000, May 2003) so this is going to be brief.
GMOLS is a computer-based algorithm that assesses engine combustion events, temperature, vehicle use, and other parameters to determine optimum oil change intervals. Oil changes are now called for when actually needed, instead of depending on generic time or mileage interval tables. Mild highway driving in a mild climate can yield change intervals of 7,000 miles (11,000 km) or more, and as high as 12,000 miles (19,000 km) for some vehicles. Short trip driving in cold weather may reduce intervals to 3,000 miles (5,000 km) or less. Most people driving a combination of city and highway will likely see intervals of about 6,000 miles (10,000 km).
When GMOLS determines that an oil and filter change is needed, the driver is notified by a Change Oil message on the instrument panel. Oil should be changed within 600 miles (1000 km).
TIP: The Oil Life System must be manually reset when the oil is changed.
How the Oil Life System is Related to Maintenance
The previously complicated, traditional normal/severe maintenance schedules required about 25 pages of explanation in the owner’s manual. The new simplified maintenance schedules can be explained in about 3 pages.
All routine maintenance is grouped into one of two schedules, Maintenance I and Maintenance II. These services should be performed alternately, each time the GMOLS message is displayed.
Benefits of GM Oil Life System and Simplified Maintenance
Benefits for the customer -- GMOLS takes the guesswork out of when oil changes are needed; the owner doesn’t have to keep track of anything. With maintenance intervals now aligned with oil changes, the customer can conveniently have both done during one service visit.
Benefits for the dealer -- Because of the typically extended oil change intervals, the customer may come back less frequently. But when they do come back, it’s for more services. The inspection and service points of both Maintenance I and Maintenance II are thorough, and are intended to keep the vehicle in good working order. They also give the technician the opportunity to locate, identify and recommend other needed services.
Benefits for the environment -- With GMOLS now installed on upwards of 20 million vehicles, if it’s used as intended, it can save almost 100 million gallons of oil in 5 years. And remember that every quart of oil poured into an engine eventually has to be drained out and properly disposed of.
In the next few months, GM is going to saturate owners with information about the GMOLS. Radio interviews, TV talk shows, magazines, newspapers, the internet and dealership kits will all be used to promote a proper understanding of GMOLS and its benefits, and to promote its proper use. GMOLS was also promoted at the National Auto Dealers Association (NADA) convention in January of this year.
How Can You Prepare?
Because of GM’s media efforts, owners are going to become familiar with GMOLS. It’ll be a good idea for you to get yourself up to speed as well.
Literature and Materials -- GMSPO is making Oil Life System promotional material available through several sources. You can download service reminder letters and maintenance schedules at . GMSPO-approved vendors have a wide selection of new service reminders including a message about GMOLS. A kit including poster, consumer brochures, and counter display is available from .
Maintenance Reminder Stickers -- Traditional stickers provided a place to write the date and/or mileage for the next oil change. To encourage use of the Oil Life System, a new maintenance reminder sticker says the Change Oil Light indicates a need for maintenance. It provides a place to check off whether the next service should include Maintenance I or Maintenance II, plus the date and mileage of the last maintenance performed.
Special Thanks to Chuck Burns
Special Thanks to TechLink 3/2004