According to Wiseco they have seen no difference whether broken in with synthetic or conventional oils, I think part of it may be an urban legend regarding using conventionals for break in. I just finished my 904 as well, I have about 400 miles on it and I'm changing to synthetic.
The good thing about conventionals for break in is that they're so cheap, and you're only putting a few hundred miles on the oil anyway so why waste good synthetic?
