I have heard my whole life that once you put synthetic oil in a motor you can't switch back to dino oil. Does anybody know why? You can buy a synthetic bleded oil wouldn't this be the close to the same if you switched back and forth?
Agreed, IMO all a myth. I've done it and seen others as well. The only thing I've seen is a leak from cover seals, but IIRC it had something to do with seal swelling/shrinking differences in the two, detergents, molecule size, etc. But that was years ago when they first became big. As advanced as oils are now I doubt it’s still an issue.