Ah, yes, the holy war of synth vs. conventional.
As far as break-in - Porsche, Viper, BMW, among others - use synth in the factory. "If it's good enough for them, it's good enough for me."
Some/most manufacturers used to use a special oil for break-in, this is rarely (never?) the case with modern motors. The primary concern would now be particulate matter coming free from friction components and contaminating the oil - many sports car drivers will replace the oil at the end of documented break-in (1200 miles in the G).
By some stretch, there are experiences that say that motors take 5-10k to really "come in". That could say that your full break-in period is 5-10k miles, at least for full performance - regardless if you baby it for that whole period or not.
I think that it comes down to what you're comfortable with. For every person who has documentative proof that one option works better, there's one with the exact opposite information. I don't think it matters enough to worry; if you have the money and are willing to spend it on synth, do it. If you don't or see no value in it, don't.
Personally, I use synth. Currently, I'm using Royal Purple - and I've been made a convert from Mobil1. My fuel economy did go up a consistent 2mpg, and my SOP ButtMeter says the car is a little stronger (see VDC thread

).
When I have the chance I rev the holy hell out of my motor. For those moments, I want the absolute maximum in protection - I have faith in synth to give that where I don't in conventional oils.
YMMV, caveat emptor, all that stuff.
Kal.