I'll relay what a mechanical engineer once told me. In grad-school, he participated in a class discussion/arguement on which was the better technique. This is what it came down to:
Hard Break In:
Great for sealing the piston rings nice and quick as well as the best possible seal for more horsepower. Horrible for everything else in the engine as burrs and crap quickly wear things before breaking off and getting into the oil. Not to mention stressing things before they tighten/loosen to operating spec. This technique is perfectly okay as long as you're consistantly tearing your engine down and rebuilding like the racers do and continuously inspect your parts for failure beginnings.
Easy Break In (Manufacturer Spec):
Good for gently nudging your engine into operating conditions and letting things settle correctly while maintaining good seal of the rings. Change your oil after the first 20-50 miles to get all the wear-in shavings out and then as suggested by manufacturer. This is the way consumers who are aiming to have long-lived bikes should break it in. Just don't sit at one rpm range too long or you'll glaze the cylinder walls and the rings won't seat properly.
He also suggested, no matter what you do, run full dino oil for about 2000 miles, then switch to a class 4 synthetic. He uses Mobil 1 and has had more than one mechanic complain that it's too hard to clean up because it's so slick :leghump , thus the reason to not use it during break in.
