ALWAYS use conventional oil during breakin period... and even though most new bikes don't even have breakins I would still err on the side of caution and go to 3k miles and then switch to synthetic... you are better off in the first 3k miles changing your oil at 1k intervals than changing to synthetic I promise you.... You will have plenty of engine life down the road to run synthetic and the factory fill on these bikes is conventional oil and many people run it the whole life of the bike so it cant be too bad...
I bought my bike (05) brand new about 3 months ago.... dealership did the first service at 700 miles and the oil came out nasty and gritty, replaced with dino... I did my second oil change at 2100 miles, oil once again came out extremely gritty and nasty... at 3k miles I'm going to get 2 oil filters and 3quarts of dino and 3 quarts of either mobil 1 or amsoil (still undecided) and going to do an oil change with the dino oil as a flush and then replace the filter again and fill with the synthetic...
I think this is a great way to go about it... but you have to consider yamaha recomends the first change at 600 i think it was (or was it even 500?) and then the next change not until 4,000 miles? I've already changed twice in half this interval and the oil come out with much more contaminants then i would ever have let stay in my crank case...
on the flip side there is possibly some evidence that changing oil too much can possibly be harmful but this hasn't been fully researched and I think that conventional wisdom that it's better to change it too much than not enough is better...
the reason you should use dino during breakin has been discussed time and time again and almost all mechanics will tell you to wait to switch to synthetic til around 3-6k miles... but I definantly wouldnt try it on my engine less than 3k.
Very few cars (bmw, ferrari, mercedes, etc. ) and I don't know of any bikes come factory filled with synthetic... but of course many people run synthetic after the fact... hope I didn't just write all this for nothing...