Engine builders of all types, sizes, purposes, from stock motors to drag/circle track open classes racing... have for decades been pounding on the table about the customer/user/owner/racer putting synthetic oils in freshly built motors. Apparently the body of evidence against is overwhelming. I've heard and read most of the arguing points and found nothing to disagree with... but I've never give it a thought to put synthetics in any of the motors I've built anyway.
By the time my motors were ready to turn over to the owner or take to the track in our cars, I had already gone through several oil changes (three during the first hour of running!) preparing each of them, including all dyno and shakeout time. I would never put synthetic oil in a freshly built motor. Never made any sense. Isn't any better oil than a good racing oil in my mind. And since oil is changed out after every event, its cost prohibitive too.
The only motors I ever put synthetics in are daily driver grocery getters (exceptions: motors I use in sub-zero temps get synthetic oil too), only after at least 5000 miles has been logged on the clock, minimum. Synthetics can benefit cams and valvetrains providing more longevity and they usually have much a lower temp pour point, else I wouldn't ever buy it. Others won't leave home without it though... and I got no quarrel with that either... if something is working for you, it don't need fixing!
