Maniac (good name for anyone on these forums),
since no experts chimed in to answer your question, I will give it a stab.
New engine bearings, cylinders, etc. have a VERY fine "scored" finish on them when brand new. During break in, these parts "run in" together and the "rough" edges are polished off, more or less. This run in sort of mates these parts together for good longevity, sealing., etc., if break in occurs properly.
Today's engines are finished much better than a few years ago and less break in is required. Most vehicles don't require break in oil and some even come new with synthetic oil.
What is break in oil? I believe it is an oil that is "less slippery" (less anti-friction additives) so that the polishing action can take place.
My new John Deere tractors come with a break in oil and they say that changing to a regular oil too soon can prevent proper piston ring sealing.
Hope this helps.