New motor break in period

[Slysc]

I've been told that a new motor should have a break in period before it gets beat on.

500 miles with 10w30 oil?

250 miles with 5w30 oil?

I guess it's supposed to seat the rings properly. What happens if I don't "seat the rings"? Will they fail prematurely? Will they never seal right?

What's the proper break in for a new motor (new pistons, rings, rods, bearings etc.)