Originally Posted by StevieC
Not sure if this was posted here yet but I did try to search first.
https://www.sae.org/publications/technical-papers/content/2007-01-4133/
Quote
The Effect of Oil Drain Interval on Valvetrain Friction and Wear 2007-01-4133
Engine oils are subjected to a series of industry standard engine dynamometer tests to measure their wear protection capability, sludge and varnish formation tendencies, and fuel efficiency among several other performance attributes before they are approved for use in customer engines. However, these performance attributes are measured at the end of tests and therefore, do not provide any information on how the properties have changed during the tests.
In one of our previous studies it was observed that engine oil samples collected from fleet vehicles after 12,000 mile drain interval showed 10-15 % lower friction and more importantly, an order of magnitude lower wear rate than those of fresh oils. It was also observed that the composition of the tribochemical films formed was quite different on the surface tested with the drain oils from those formed with fresh oils. The objective of this investigation is to demonstrate how the friction and wear performance changed with oil drain intervals. A fleet of three vehicles was run in Las Vegas and oil samples were collected at various drain intervals from 3000 miles to 15000 miles. As in the previous study, the results showed that the aged engine oils provide lower friction and much improved wear protection capability. These improvements were observed as early as the 3000 mile drain interval and continued to the 15000 mile drain interval. The composition of tribochemical films formed on the surface with the 3000 mile drain interval is similar to that formed with the 12000 mile drain interval as seen before. These findings could be an enabler for achieving longer drain interval
although several other factors must to be considered.
I underlined another bit that's pertinent...
Per the paper, they took samples of oil from the taxi cabs, and subjected them the a test in a simulated valvetrain.
Entirely predictably, the already degraded oil filmed tribofilms more rapidly, and reduced wear and friction...when introduced to the test apparatus.
That doesn't say that leaving the oil in longer, with an
already established tribofilm will result in less friction and wear...just that the used oil introduced to the test apparatus did.
As to the "other factors"
First test oil had a TBN of 3.2, TAN of 7.6, and had thickened from 8.6 to 15.9 Cst...it was clearly unserviceable at around the half way mark...but yes, it performed well when introduced to the test rig.
Second oil thickened markedly also, (8.7-13.3, TAN/TBN crossover between 5,00 and 7,500, had 150um of Fe at 5,000 miles...which disappeared (into sludge ???) by the end of the test.
Third oil, 8.3 to 17.1 Cst.TAN/TBN crossover between 7,500 and 10,000 miles. and also played "hide the iron" towards the end of the test.
It showed that fresh oil on a fresh shim produced markedly more wear than either used oil on a used shim, of fresh oil on a used shim (already established tribofilm), with used oil on a used shim being slightly lower than the "oil change" example.
Not an attack Stevie, but this paper has been the most misrepresented paper on the site, and not many people have anted up the $25 to read it...heck, CATERHAM waxed lyrically on how it showed reduced engine wear clearly due to the thinning of the oil, which made it lubricate better...a posit that he defended vehemently, when it was diametrically opposite to what's clearly stated in the paper.
So how do I use the paper ?
I change the oil one weekend, leaving what's in the filter...then the filter a few weeks later. The filter introduces "activated" oil to the new charge, and takes out any grit in the new oil (it's there), while the filter is working at it's best (filtes more junk the more junk that's in it).
Am prepared to sell "oil pre-conditioner" by the litre to any interested parties