Well, nothing happened to it as a result of using synthetic oil. It's just not worth it, as mineral oil is ideal and cheaper than synthetic.
My last engine blew up (dunno why), so I got a new (second hand) one, and got it working in the bike. But it's gone through about 3 liters of oil in about a month. I started topping it up with synthetic, but it just keeps dissapearing. After that, I was told that theres no point using synthetic anyway, as mineral is best for it. So I wasted my money on synthetic when mineral would have been sufficient, and it just disapeared anyway. The problem isn't that I was using synthetic oil though. Any oil I put in it just gets guzzled away. I topped up, done 100 miles, and then I had to put anoher 270mls of oil in it.