Bob Donalds J ust as with the engine in a brand new car, a rebuilt engine needs a break-in period to insure long life and low oil consumption. The new rings, lifters and valves have to find their place in the scheme of things. The new rings and cylinders generate a lot of friction during the break-in. The bore has a cross-hatch scratched into it, and dragging rings across the freshly honed bore creates initial frictions that are only there until the rings seat. Three rings on four pistons moving up and down real fast, even at low RPM, create a lot of heat in a new motor, and this is extra heat the new motor has to contend with.
Oil is a very good heat exchanger as well as lubricant.
I change the motor oil twice in the first thousand miles. The used oil comes out looking like metal flake because it has the residue from the rings and cylinder walls. You will see a decrease in crankcase pressure when the rings finally seat properly. If lubricated properly, these pieces will work-harden. If they become dry, they will scuff and wear out rather than break in.