Breaking in with synthetic is OK.!@!!!!! In fact, if I bought a new one, that is all I would run, for I don't like the thought of crosscontamination. I think you should run on the same oil all of the time.
I built the enigne on my 16v and used ONLY Mobile One. I broke it in on it and changed it after 1K miles. It is now running fine with no oil consumption.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS