I think longston and dakota have already answered your question better than I could have.
I will correct myself, as longston pointed out, tolerance is not exactly correct, so I will use the word precision. All of the fitted parts are made to much more precise dimensions.
As for materials, an example is the cylinder material in the M112 engine, the new V6 and its sister that has two extra cylinders. These cylinders are an alloy that is somehow used with the aluminum cylinder block. I saw one of these engines apart at 50,000 miles, it was apart because of an accident puncturing the oil filter cannister. These cylinders had the sharpest, freshest looking crosshatch you could imagine and very little deposits at the "ridge". These engines use a low tension ring that is sealed immediately.
Also, these engines have roller cam followers. As was very well put by one of these gentlemen, the cam and lifters are critical during start up. Most aftermarket camgrinders recommend proper assembly lube and the first twenty minutes not letting the RPM fall below about 2,000. If you get past that, you're in great shape. With the roller cams in these engines, you could probably not even lube the camshaft at startup and it would get enough splash quick enough that no harm would be caused. I'm sure you won't find ANYONE willing to test this theory, however.
As longston said the additive package in modern oils is far beyond anything we had as recent as twenty years ago.
All this adds up to an engine that for all practical purposes just doesn't require break in.
[Edited by LarryBible on 02-01-2001 at 07:37 AM]