Al, I don't believe this is as straight-forward as some would lead us to believe and would like to see more discussion.
What about a rebuild by the local speedshop?
What about a chevy crate motor?
What about a new Japanese rocket bike?
What about a new car with an 20yr old engine design?
What about a new car with an update of an old engine design?
To me each engine is specific because it will have unique differences in parts and construction. Answering the break-in question either way without knowing those factors is something of a shot in the dark.
-Can we lay out the reasons synthetic oils were originally not recommended for new motors? (ring seal, etc)
-Can we list differences in engine design and construction that would affect this, and how they've changed? (machining tolerance, factory break-in, etc.)
-Do we know if modern dino oils have similar problems where break-in is required? (just group 3's, or anything with good high temp EP?)
Seems like an important sub-specialty of engine lubrication. I'd very much like to understand more about it.
David