They don't. Plenty of engines, even run on cheap bulk oil changed regularly end up looking fantastic at high mileage. We've had plenty of "post under your valve cover" threads here.
Contaminants, such as broken down VII molecules, soot and the like are held in suspension by the detergents and dispersants that make-up the lubricant's additive package. It is when the contaminant holding capacity of that lubricant is exceeded that these products plate-out on surfaces, most often those that are cooler and low flow. As long as the lubricant is changed before this threshold is reached, things will stay clean.
If you have an engine that is extremely hard on oil and rapidly breaks it down, it doesn't matter if you flushed it 5x before an OCI, if you exceed its contaminant holding capacity, deposits will form as a result.
If you have an engine that is extremely easy on oil, as long as the lubricant is changed at a reasonable interval that is below its threshold for contaminant holding capacity, things will remain clean.
Wear is more to do with duty cycle and design, not oil change procedure. That's why OTR diesels rack up obscene mileage and why the highest mileage gas vehicles on the road are typically seen in some sort of courier, limousine or taxi service, none of which experience exotic oil change rituals but all of which have long periods of stead-state operation with oil up to temperature.
Does that mean we shouldn't strive to do what we feel is best for our equipment? Absolutely not, but I think it important to acknowledge that most people will never accrue the type of mileage that vehicles maintained far less rigorously will see in service more conducive to it.
Most people never come to these forums, rarely keep a new vehicle far beyond the factory warranty or yet beyond 10 years and only care about when the OLM tells them to change and where the best price on the next quick oil change. My method is directed at enthusiasts and those wanting to keep their vehicles for long periods of time.
My wear rates are often 1.5-2x higher when I leave 15-20% of the old in after a spill and fill. I have seen my fuel dilution start off at 1-2% when I do not flush the old oil out and at levels above 4% I see a similar increase in wear. I have seen my iron wear levels start off at 5ppm rather then 0 ppm when I leave the old oil in. That residual wear metals do have an affect on future wear. The viscosity thins out much quicker, and TBN is reduced much quicker when I leave the old oil in.
My wife and I follow different oil change methods. She changes every 3k miles in all her cars, with the current recommended API oil that is available at the most convenient oil change service center.. Her car is mechanically sound with no oil consumption, and no fuel dilution, mixed driving and 180k miles. Her camshaft cover and valve train has heavy varnish and deposits. I run 2-3x longer drain intervals, with high fuel dilution and premium synthetic oil and Full Volume oil changes and my valve train has zero varnish and sludge and looks like new at 150k.
I have seen many of the same examples as well on both ends of the spectrum.
How does one know when when the dispersant and detergents holding capacity or threshold is reached. I rarely see much of change in additive levels even when I triple my OCI. Oxidation and nitration only increases small amounts. The dispersants do not help with fuel dilution or wear metals such as iron, aluminum, lead, tin and other metals correct. I think they have some affect on free copper from coolers based what I have read.
I would like to see some actual comparative studies between short volume oil changes vs full volume oil changes with respect to wear rates, and lube performance with both normal and extended drain intervals. Many interested parties would cry at the thought of the so-called wasted oil with FVOCs. Wynns Corporation that makes coolant and transmission fluid exchange machines and used by GM dealers also made an engine flush machines that uses clean oil . I have seen these services in Japan and Korea when I lived there. It never took off or was adopted in the US, because so many believe all the oil drains into the pan upon shutdown, and the minute amount still in the engine has no affect on the new oil or engine. My first engine I rebuilt was a Ford 289, much different design and a lot of oil would drain. These new engines with VVT, DOHC designs, turbos, coolers retain far more. I have looked and have not seen any comparative studies, but I do agree with Machinery Lubrication articles on residual oil and short volume oil changes, and their affects, as I have seen similar affects in my own vehicles.
We can probably agree that keeping the oil clean and for longer periods, improves performance, improves oil performance, reduces wear and extends the life of the equipment. I do full volume fluid changes on my transmissions and they have all lasted 250k miles or more.
