Well it’s also decades of passed down information and at times conflicting information.
working in that test cell for cummins, engine break in was a hotly debated topic everytime until Cummins finally put a policy in place. Every engine is brought to operating temp, then broken in. Then as sample, every 100th engine is started up, rev’d to 1800 RPMs where Max torque is stock, then sat there for an hour, if it was still running after that, the batch was determined good. If not, it was torn down and sent back to engineering