My previous oil had a mix of MMO in it. But, I've been second guessing that move, because I wonder if a certain amount of varnish is actually a good thing. I say this because after adding MMO, my tappets became much cleaner, but I noticed that brown varnish was filling in the micro-scratches on the surface of the tappets and the cam lobes. So, it got me wondering whether motor oil forms certain films (which might include a certain amount of varnish) that help against metal-to-metal wear.
Does anyone have this article?
The Effect of Oil Drain Interval on Valvetrain Friction and Wear
"Engine oils are subjected to a series of industry standard engine dynamometer tests to measure their wear protection capability, sludge and varnish
"...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."
It seems that tribochemical films (which I'm assuming possibly includes varnish films on surfaces) help protect against wear.
And, there's also these articles:
Formation, structure, and properties of lubricant-derived antiwear films - Willermet - 2006 - Lubrication Science - Wiley Online Library
Does anyone have copies of these articles? It seems MMO and other additives might remove antiwear films, but I'd like to read the articles for their conclusions....