I've heard that it's best not to use
synthetic on a new engine until you have a couple hundred miles on the odometer. I think it has something to do with allowing the rings to seat.
I broke in the engine, changed the
oil & filter, drove it for 500 miles and switched to
synthetic with a new filter again. This may have been overkill, but I wanted to make sure that it was free any lubriplate and cam lube.