I have to disagree with the idea of a hard break-in.
FIRST: I think the engine manufacturers must know something. My 2002 toyota book says for the first 600 miles:
-Don't exceed 62 mph
-Keep rpm between 2000 and 4000 as much as possible
-Don't accelerate hard
-Don't lug the engine
-Don't tow
SECOND, and more importantly: Unless it is an MS or some other specially prepared engine, there are always parts to be molded to their matching parts during the first "X" miles. That is why on analysis after 100 hours of operation on new engines I see:
-Fairly high levels of IRON
-High levels of Copper, Tin, and Lead: Soft metals used in the final layers on bearings to wear immediatly and leave a full smooth surface where oil can flow and give the hydrodynamic lubrication that Bob explains so well on this site, and boundary lubrication can work evenly.
If we have uneven surfaces, and push hard on the accelerator, the pressures will cause contact on the ridges and cause more heat, faster wear of those materials, probably in larger particles that might damage other things.
This is also why I believe in an early oil change. Get that stuff out of there.
Terry probably has more analysis like this, but here are my most recent two examples of those levels: Both CASE Diesel Tractor engines with 100 hours (will normally run 300 to 400 hours between changes)
Iron: 28 ppm -- 26 ppm
Copper: 56 ppm -- 44 ppm
Lead: 7 ppm -- 5 ppm
Chrom: 2 ppm -- 2 ppm
Alum: 3 ppm -- 4 ppm
Silicon: 19 ppm -- 23 ppm (probably from seals)
Theories are easy to put on the internet, but I think analysis helps define things.