I used to believe that no synthetic thing but I have broken my last few engines in on my usual Belray MC-1 with no issues. With a 4-stroke I have definitely experienced problems. This is a different issue, but I have had break-in seizures using 32:1 mineral oil but none using 50:1 synthetic. However, after saying all that, it won't hurt one bit to break it in using non-synthetic oil if that is the preference. What does the dyno say? Cam.