Of course, not mixing it in the crank like a 50-50 mix of antifreeze, I don't see how going from one type to the other will make the seals leak. Syn oil is more apt to leak around seals because it's thinnner. The amount left in the oiling system and general internal residue isn't enough to make a difference as far as the definition of mixing the 2 oils. And I don't like dirty oil to begin with, so whatever oil is in the crank will never live there long enough to make a difference.
