FWIW I can relate what my experience has been from 35+ years in the engine industry.
I cannot think of any serial engine manufacturer which uses anything other than a regular service spec oil as initial factory fill.
For development work we always carried out a "break-in" procedure, the details depended on the manufacturer but all were of a similar concept. Start at low speeds and light loads for maybe 15mins to warm up thoroughly (and allowing checks of system functions etc), then up to perhaps 1/4 speed light load, and gradually ramp the load up to perhaps 1/4 load (BMEP) over maybe 15mins, hold for maybe 15mins. Reduce to light load and increase speed (maybe 1/2 max rpm), hold, ramp up load to 1/4, return to lower speed and ramp up load to 1/2, return to mid speed and ramp up load to 1/2, reduce load and increase speed to 3/4 etc etc.
It is usually this sort of sawtooth type sequence, gradually and progressively increasing the speeds and ramping up loads until you reach max power speed and load after a few hours. It would then be held at Pmax for perhaps 15mins. The whole process is typically 10hrs, sometimes a little less.
At the initial stages the speed is the critical parameter, load is not so important to begin with. The break-in is principally for the "piston/bore interface", i.e. bore, rings, skirt. The bore surface technology is sophisticated these days, the preparation in manufacture to achieve the optimum bearing area ratio (running surface area vs oil retention volume) is very carefully engineered. However good the processes, there is always an "excess roughness" which is derived to achieve a controlled wear process between the rings and bores to optimise the running conditions after a short bedding-in time. The peaks of the material surfaces will project through the oil film and will be worn away, the risk initially is that with high pressure contact at these peaks the friction can result in micro-welding and tearing of the surfaces, which does not subsequently "heal". It will polish over, but it doesn't give the optimum surface. The idea is to achieve a running surface between Mr1 and Mr2 on the picture below to maximise service life with an optimised oil retention volume. The Rpk is the height of the peaks (asperities) which are worn off during a break-in to get down to the straight line part of the diagram.
This is the sort of thing
https://www.diva-portal.org/smash/get/diva2:820446/FULLTEXT01.pdf
Engines are engineered to be "tolerant" of abuse but this doesn't necessarily result in the optimum conditions for maximum service life. There is usually a piston scuff test where an engine will be built with minimum bore clearances, it will get the minimum build process running (often 2mins) then will be taken directly to max power. This will be the worst case scenario and it must be able to survive it.
Car manufacturers don't specify any process because it can cause confusion and dispute, so they engineer the product to tolerate the worst and hopefully be treated well. Usually new owners will drive a car more or less reasonably anyway. If you get a new engine the best thing is to use the engine progressively, the rpm being the most important. Start using modest speeds, increase load a bit, then work up the combinations of speeds and loads. On the road I would suggest 20hrs or 500mls is ideal to get to the point where you are using full throttle and near max revs on regular occasions, you don't do the engine any favours by being too gentle at this stage, they are made to run at max power and you will not harm it.
There is some degree of truth in the "Italian tune-up" theory, the worst possible conditions are when an engine is used for repeated low speed short runs, 2mls to the shops once a week. Valve rotation (to even out wear and sealing) usually starts around 2500-3000rpm, and I always try to give an engine a short time at 3k or more (once fully warm) especially if I've been driving in traffic at low speeds a lot.
