That's really more of a myth than anything. Dodge even puts Mobil-1 Synthetic into their cars out of the factory!
That's what is recommended by the people making synthetic oil, they print it right on their bottles. I'm aware of new cars having synthetic right from factory, definitely conflicting.
Which oil manufacturers recommended that? I have a good friend who is a Jaguar Master Tech, he and all the techs there swear by, "If you ever put anything other than 100% Synthetic oil in an engine, it's useless to ever put in synthetic oil. Use a synthetic blend if you've ever put a non synthetic oil in a engine."
Roller cams may have a big thing to do with whether or not you use a conventional/synthetic oil for break in. I'm not sure though, it's just a guess.