Your best bet is to just go with what the manufacturer tells you. IF no stipulations are made in the owner's manual (be sure and ask the dealer too)change the break in oil as soon as you want with whatever you want. If you're told to leave the break in oil for a certain period or not to use synthetic for a certain period, then follow those instructions and begin using syn after the specified time. When I say ask the dealer, I mean the service dept, not a salesperson unless it is clear he/she is a well-informed motorhead. Seotaji is probably on the right track with Porsche...I believe they do run in their motors before final vehicle assembly and the others (Corvette, Aston Martin, Viper, AMG, I believe, etc) may have special procedures as well.