I've heard it said that one should not use synthetic oil during break-in, because it can impair proper wear-in of the rings and other moving parts.

Well, my daughter (lucky gal) just bought a new 2007 Pontiac Solstice, and lo and behold, the car comes equipped with synthetic oil from the factory. Corvettes, Vipers, and many others do too!
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So, this must mean that, at least for some new engines, that it's perfectly OK to run synthetic oils from day one. Does this apply to motorcycle engines? Hard to say. I assume that it depends on the manufacturing methods used, but these days, I assume most all automotive engines (including motorcycles, of course) use similar methods and are manufactured to close tolerances and require less rigorous break-ins than did engines made when I was a wee lad. So, the answer is: perhaps.
Enough of my uninformed rambling! What do the oil manufacturers say?