It depends on the type of break in oil used as to how long it stays in there. Over the years Ive seen 10 minute oil to 3,000 mile oil as break in oil, for a while there was synthetic break in oil. It just depends what that engineer was trying to accomplish, and many of them were only trying to accomplish sales for $$ in their pocket even if it was at the expense of your engine’s life. It wasn’t until hundreds of people started sharing experiences online that the synthetic was learned by enough people that it went away.
Some folks are surprised to learn about the powders rubbed onto cylinder wall and assuring ZERO oil on it for the gapless rings for instance. But it was used only on certain types/ applications. Taking the time to learn which method it best for specific application is where most folks get wrecked. The way a diesel pickup that just gets daily driven like a passenger car gets done is different than a drag racing engine- even if they are both a 6.5 in the same year truck.
Having the pistons ported and running ported rings is awesome for the track, not for going 300,000 miles. They require different cylinder wall finish (which gramps never heard of different finishes) requires different break in oil and different process.
And NO, you don’t want any of the debris flowing around with the oil everywhere.
That’s a theory the guy had.
Most of what is being broken in. Is the rings seating to the cylinder walls. A little of the cylinder walls and little of the ring material gets worn away to get the microscopic fitment.
Nothing nothing on piston, pin, rod wears away. Microscopic levels only of the rod, crank, and cam bearings will wear away- but honestly the less the better. A y wear there is actually undesirable.
The unfortunate new engine break in damage actually the bearings trapping any of the other metals that did wear away and get floated in the oil & not yet caught by the filter- can get to the bearings and actually embed into the soft bearing material and do long term and even engine ending damage to the crank, rods, cam.
What else wears: a new timing chain /gears & timing gears get any microscopic wear into them from not being manufactured perfectly. Same for cam gear to oil pump drive (or old 6.2 vacuum pump that was there). Where the lifters ride the cam lobes, the lifter rollers themselves, these start some microscopic wear. Both ends of pushrods, both ends of rocker arms & top of valves. Mechanical lift pump pushrod & arm for those with the mistake of still using them. Again all get the tiniest amount of microscopic wear during break in.
But this is misunderstood by many - NONE of these other items wearing or breaking in is a good thing. Having accelerated wear in all of these points is a negative effect.
Ideally everyone would get an oil accumulator and the bearings & such would never experience any of this because of the pre-charge of oil before every start up including the break in debris being displaced- not used like a rubbing compound as the guy described to you.
The metal to metal contact that is under force (like rocker arm to valve top) is at the load it’s entire life and has no film of oil separating them, it gets passive oiling to help ease the wear- but all wear hear is bad. No wear pattern is helpful.
The bearings should theoretically never have any wear. That super tiny clearance should retain enough oil to help until the oil pump has fed enough oil to keep the bearing surface away from the spinny metal item next to it. Almost all bearing wear is at start up before the oil pump can start doing its job. Hence the oil accumulator comment. Or similar is an auxiliary electric oil pump you turn on before starting. Having these will almost fully eliminate bearing wear. If you ran one of these, you could pull apart an engine with well over 300,000 miles and have nearly untouched bearings. The only wear and damage would come from debris floating around in the oil- much like the rubbing compound theory that guy was thinking of.
It used to be a thing where people would actually guy an oil filter- or the even older engines that had completely external oil filters-it would be bypassed during break in. All in the belief of the theory deemed rubbing compound. And it turns out, these engines would seat the rings in the same amount of time as ones with filters intact- but would infact suffer sorter lives, loose compression sooner from wall scoring, damaged bearings,etc. Alas- “Great Gramps was the best mechanic ever so we do what he did”. Just remember many great gramps had engines that never had any oil filters and if they couldn’t see chunks or sludge in the oil… it was fine to keep going.
The very best break in process for any engine regardless of long life vs race is a dyno or dyno sim machine. But that’s $$$ so most don’t do it.
