Have done that Shep.
I was talking to someone with similar engine and driving style/climate
etc. and his came on between 8,000 to 12,000 kms.
A lot of sites claim that 7500 miles is programmed as the upper limit.
I've done oil changes without resetting the light and it still hasn't
I was just wondering if there is a procedure to check it out.
My wife's Buick has one of these idiot lights, and I have never researched
way it functions. Consequently, I ignore it, and change oil on schedule.
Can any of the pro's on here give a quick discussion of the theory behind
oil change lights? Much appreciated.
One thing I can touch on and clear up.....the GM oil life monitor
operation and my statement that ZDP (or ZDDP as you tend to call it
here...most of the API literature just sticks to ZDP so I tend to use
that) depletion is the basis for oil deterioration.
My spelling is poor but ZDP stands for zinc dialkyldithiophosphate
which , as it sounds, is an anti-wear compound comprised of zinc and
ZDP is dispersed in the oil so as to be at a potential wear site if a
surface asperity happens to break thru the oil film thickness causing
the dreaded metal-to-metal contact. A molecule of ZDP must be present
at that moment to prevent microwelding at the contact site which will
cause material transfer, scuffing, scoring, wear and catostrophic
failure. The concentration of ZDP in the oil will determine if there
is ZDP present to work it's magic. The greater the concentration...the
more likely a molecule of ZDP will be there...and vice versa.
By nature, ZDP is sacrifical. As ZDP is "used up" at a wear site to
prevent micorwelding the concentration of ZDP decreases.... So...if
you measure the ZDP concentration in engine oil in a running engine it
will decrease at linear rate based on engine revolutions. Any given
engine has a certain number of high potential wear areas where metal-
to-metal contact could occur due to reduced film thickness and/or
surface asperities....areas such as rubbing element cam followers,
distributor gears, rocker arm pivots, push rod tips, etc...... The
more of these areas the more ZDP depletion. The more often these
features come in contact the greater the ZDP depletion. That is why,
generally speaking, ZDP concentration in the oil, for any given
engine, will decrease at a fairly linear rate when plotted versus
cummulative engine revolutions. The more times it turns the more
contact the more chance for wear the greater the depletion. This is as
much of a fact as I could quote ever and is really not speculation or
anything. It is proven beyond a shadow of a doubt in many studies.
That is why it is ONE of the basis for determining oil life remaining
and why it is THE basic premis of the GM oil life algorithm. It is
only ONE of the things that determines oil life...but it is the one
thing that can be tied to engine operation in a linear fashion and
estimated very accurately by accumulating engine revolutions via a
The GM engine oil life monitor counts engine revolutions and
accumulates the number for the basis of the oil life calculation. It
then adds deterioration factors for operating temperature, start up
temperature, soak times, ambient, coolant temperature, etc... There
are a LOT of factors that "adjust" or affect the slope of the
deterioration but the fundamental deterioration is traced back to the
ZDP depletion that is inescapable with engine revolutions. The
specific rate of ZDP depletion is readily measurable for any given
engine so that is the fundamental item that is first calibrated for
the oil life algorithm to tailor it specifically to that engine.
You would obviously like to get the oil out of the engine before the
ZDP concentration gets so low that it is ineffective at being at the
right place at the right time and preventing engine wear so that
becomes the long term limit on oil life for that application.
The other things that determine oil life such a acid build up,
oxidation, petane insuluables such as silicon from dust/dirt, carbon
or soot build up from the EGR in blowby, water contamination, fuel
contamination, etc.... are all modeled by the multipliers or
deterioration factors that "adjust" the immediate slope of the line
defined by the engine revolution counter as those items can be modeled
in other ways and accounted for in the immediate slope of the ZDP
The algorithm was developed over the course of many years by several
lubrication experts at GM Fuels and Lubes, spearheaded by Doctor
Shirley Schwartz who holds the patents (with GM) for the algorithm and
the oil life montitor. I had the luck of working directly with Dr.
Schwartz when the idea of the oil life monitor first progressed from
the theoretical/lab stage to real world testing/development/
validation. There were fleets of cars operated under all conditions
that deteriorate the oil life for any and every reason and , thru oil
sampling and detailed analysis of the oil condition, the algorithm was
developed, fine tuned and validated to be the most accurate way
invented yet to recommend an oil change interval by. As just one
example, I have seen cars driven side-by-side on trips, one towing a
trailer and one not, for instance, to prove the effectiveness of the
oil life monitor in deteriorating the oil at a faster rate just
because of the higher load, higher average RPM, higher temps,
etc...and it works flawlessly.
The oil life monitor is so effective because: it is customized for
that specific vehicle/engine, it takes everything into account that
deteriorates the oil, it is ALWAYS working so as to take into account
THAT INDIVIDUALS driving schedule, and it tailors the oil change to
that schedule and predicts, on an ongoing basis, the oil life
remaining so that that specific individual can plan an oil change
accordingly. No other system can do this that effectively.
One thing is that I know personally from years of testing and
thousands of oil analysis that the oil life algorithm works. There is
simply no argument to the contrary. If you don't believe me, fine,
but, trust me, it works. It is accurate because it has been calibrated
for each specific engine it is installed on and there is considerable
testing and validation of the oil life monitor on that specific
application. NOt something that oil companies or Amsoil do. They
generalize....the oil life monitor is very specific for that
Oil condition sensors in some BMW and Mercedes products are useful,
also. They have their limitations, though, as they can be blind to
some contaminates and can, themselves, be contaminated by certain
markers or constituents of certain engine oils. Oil condition sensors
can only react to the specific oil at that moment and they add
complexity, cost and another potential item to fail. One other beauty
of the GM oil life monitor is that it is all software and does not add
any mechanical complexity, mass, wiring or potential failure
There is considerable safety factor in the GM oil life monitor.
Typically, I would say, there is a 2:1 safety factor in the slope of
the ZDP depletion curve....in other words, zero percent oil life per
the ZDP depletion is not zero ZDP but twice the concentration of ZDP
considered critical for THAT engine to operate under all conditions
reliably with no wear. This is always a subject of discussion as to
just how low do you want the ZDP to get before the oil is "worn out"
if this is the deciding factor for oil life. We would tend to be on
the conservative side. If the oil life is counting down on a slope
that would recommend a 10K change interval then there is probably 20K
oil life before the ZDP is catostrophically depleted....not that you
would want to go there...but reason why many people are successful in
running those change intervals.
Please...NOT ALL ENGINES ARE THE SAME. The example above is an
excellent practical justification of why you would want to add EOS and
change the 15W40 Delvac in the muscle car at 3000 miles max and yet
can run the Northstar to 12500 easily on conventional oil. You must
treat each engine and situation differently and what applies to one
does not retroactively apply to others. This is where Amsoil falls
short in my book by proposing long change intervals in most everything
if you use their oil. It just doesn't work that way. You can run the
Amsoil to 12500 with no concerns whatsoever in the late model
Northstar because even the oil life monitor tells you that for
conventional oil off the shelf. Would I do that to the 502 in my 66
Chevelle...NO WAY. Amsoil says I can though. Wrong.
There are entire SAE papers written on the GM oil life monitor and one
could write a book on it so it is hard to touch on all aspects of it
in a single post. Hopefully we hit the high spots. Realize that a
GREAT deal of time, work and energy went into developing the oil life
monitor and it has received acclaim from engineering organizations,
petroleum organizations, environmental groups all across the board. It
is not some widget invented in a week and tacked onto the car.
The oil life monitor is not under the control of a summer intern at GM
Powertrain per an earlier post....LOL Not that a summer intern wasn't
compiling calibrations or doing a project on it but is under control
of the lube group with a variety of engineers directly responsible
that have immediate responsibility for the different engine families
and engine groups. The idea that a summer intern was responsible for
or handling the oil life monitor is ludicrous.....LOL LOL LOL
The following is the response of a GM engine development engineer to a
motorcycle forum on extended mileage oil changes. The link to the
was posted on VV months ago. I found it so interesting I saved a
Read with an open mind and form your own opinion about your favorite
and oil change habits. Only the GM engineer's responses to questions,
converted to plain text format, are included. The GM engineers
experience and credentials are in the first few sentences of the last
paragraph. It's taken special handling to get this long message
thanks to vv-help. I've got the entire original in pdf format for
The real issue with the ZZDP is that modern oils have had the content of
these additives reduced to about 1/8 of what they were to prevent catylytic
conv damage. This is the main reason racing engines using flat tappet cams
are being run in on 15/40 diesel oil for the anti wear/scuff package that is
still used in diesel oil.
You know what? I'm not aware of anyone ever complaining
about the operation of the oil life monitor at our dealership.
So as a result, the idea of checking the system has never
crossed my plate. I'll have to check into it.
I appreciate your post. I had no idea that they had put so much effort
into this system. My "idiot light" comment was an assumption and is
One more question: Several have posted that they have not seen many
incidences of the system being suspected of not working. But I would
also assume that, if one nears the oil change interval recommended in the
manual and if the system warning doe not trigger, one should consider
the time or mileage interval to override the warning light??
GM says in their owners manuals to change the oil and filter within
at least a year, no matter what the oil change monitor says. They also
say that if you are unable to determine if the oil life monitor is either
working, or was reset inadvertantly between oil changes, to revert
back to the 3000 mile oil change interval, then confirm that the
monitor is reset after that oil change.
Many thanks, Ian.. There is not a GM in our immediate future, but wife's car
monitor. At this stage of mileage, warranty is not an issue/
We pick up our new car on Friday.
By the way, one heck of a competition in Malaysia! Kudos to Team McClaren.
Correction. It does NOT say to "revert back to a 3000 mile interval"... GM
does NOT recommend a 3K oil change cycle.
rather, it says, have your oil changed within 3,000 miles of the time you
discover that you didn't reset the monitor.
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