I made a bet with my boss in 1984 (when I was a young mechanic, not a chemist), and my dad later that day - I will never change my oil (No, not the filter either) again, just add when needed, and not suffer any oil related issues. In that time I’ve driven several vehicles (with and without fuel injection, but not diesels) 100,000’s of miles, most of the vehicles had nearly 100k when by the time I got them, and I drove some way past 200k. One was a retired Public Service(the old electric company around here) van that probably had the odometer roll twice before I got it at an auction and used it as my construction van during the 90’s for 70-80k miles. The tranny was rough when I got it and nearly dead when I donated it for scrap, no radio, heater, windshield wipers, interior lights, seat fabric, muffler, and much more in the NOT category, but the motor always started and ran strong, it passed yearly emissions testing(so did all my other cars). A few years ago I bought a 2003 Dodge Caravan with ~70k, it now has over 100k, although it’s not that many miles, it’s the only car I have that has MPG readout and it says I’m still getting about 19mpg, the same as when I bought it.
So I would say that I have yet to lose that bet, the only mechanical problems I’ve suffered have been outside the engine, usually something attached to a spinning belt, or wheel. Neither my boss or dad ever paid up (I forgot what is was), they’re gone now so I can’t continue to rub it in. In defending my reasoning to others over the years I extended the bet to many of my things that have oil, like my air compressors, lawn mowers and generators. I have an air compressor holding ~150 psi that has been turning on at least a couple dozen times a day for the last 22 years, no oil change, no problems - ever.
Do I think that all the guys with the extensive explanations the perils to be had by those that don’t change their oil (with the good stuff of course) are wrong about what happens to mechanical parts? No, well maybe. Am I just the luckiest guy in the world? Or was I right in 1984 when I asked - what does oil that has been in the ground for millions of years at high heat and pressure, oil that was refined by boiling it at temperatures way higher than will ever be seen in an engine, what does this oil break down into? I have never seen oil “break down” into something that is not slippery, so why is changing oil any kind of insurance?. Now, if you drive a race car (high rpm and loads on the motor parts) or if your engine gets overheated, or something like coolant gets in the oil, then there might be problems, and I suppose I wouldn’t apply this to things like a gearbox on a windmill until I had some experience with that sort of thing . So yes, I’m sure it’s important to change your oil in your car, if you are planning on keeping all the rest of the parts for eternity, otherwise my experience says your just wasting your/their money, boss;)
It’s been almost 30 years and although I have never seen it, and I’m still not a chemist, I’m still wondering, and asking - what should I expect my oil to “break down” into that is not slippery?
As far as changing oil for sake of the filter and the bits of metal that ground off when a cold engine starts, I say leave the bits in there, after the engine warms up they will be re-deposited where needed, that’s just like the theory behind engine compression boosting products, isn’t it? Some of those products have suspended metal bits that are small enough to flow through the filter, but that fills scratches in the cylinder walls, don’t they? It seems like oil that has been flowing around for a while would have a give-and-take type relationship with the engine, and changing the oil is removing all the bits that are like salve for the motor, putting in an aggressive astringent that soaks up a whole new layer of engine bits that are flushed away a few thousand miles later. That’s why even well maintained motors start to loose compression after 100-150k or so, maybe it’s worse the more you change? This is all just my theory, I’m still testing it (not the changing more part). But really, why am I not having any problems?