break in on motors is almost as controversial as break in on a rifle barrel
Polaris on their ATV's run synthetic from the factory, i have one, put alot of miles on it, runs great....
i think alot had to do with tolerances of vehicles, much tighter tolerances today. I think what is more important is the rpm's during break in, that you vary the rpm's, and that you run the rpm's up through a range
i used to race motorcycles, we built alot of motors, we varied the rpm's and even ran them up to redline during break in, we just never held them constant, but we ran rpm's up to get a good seating of the rings to the cylinder walls....