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 phosphorus.
ZDP is dispersed in the oil to be at a potential wear site if a surface asperity happens to break through the oil film thickness, causing the dreaded metal-to-metal contact. A molecule of ZDP must be present at that moment to prevent micro welding at the contact site which will cause material transfer, scuffing, scoring, and wear and catastrophic 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 sacrificial. As ZDP is "used up" at a wear site to prevent micor welding the concentration of ZDP decreases, So if you measure the ZDP concentration in engine oil in a running engine it will decrease at a 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 cumulative 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 premise 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 counter.
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 an acid build up, oxidation, petane insoluble’s such as silicon from dust/dirt, carbon or soot build up from the EGR in blow by, 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 depletion line.
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 monitor. 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, through 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 application. 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 mechanism.
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 catastrophically depleted, not that you would want to go there but the reason why many people are successful in running those change intervals.
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, and environmental groups all across the board. It is not some widget invented in a week.