The piston is made on a lathe, if you have a fresh piston and you zoom in on the surface you will see the lines made by the toolbit on the lathe. This surface is ideal to use for the break-in



Zoom in on this pic:
http://www.euronet.nl/users/tooms/ftp/DSC_0024.jpg
you will see sthe small lines.....

With less oil and high rpm the tips of the rough surface of the piston will burn and snap off, that will create small noticeable particles. With lot of oil, a slow movement and a controled temperature the surface will be polished giving a dirt in the oil that small that it will not create scratches