I think the only downside going over to synth oil would be if you have bad gaskets. Old bad gaskets = oil leak of some sort.
It's not that the oil cause the leak but the old oil was just too thick to go through.
But this could be all crazy talk. It's just what I heard.
Synthetic oils may be better than regular oils, but are more expensive. In reality, if you change the oil often you can save a lot of cash by using regular oils. I don't think that one type of oil extends engine life over the other.
I switched to Mobil 1 (synthetic), from Castrol GTX regular oil on my Forester, but only because a gallon of synthetic oil costs around $22.00 at Walmart. Otherwise I would have continued using Castrol GTX. I have been using regular oil on a 1987 Civic Si I drive to work each day, and the odometer has over 253,000 miles (I had thought it had 257,000 already! I wonder if it's moving backwards?) I have used Castrol GTX, Penzoil, Quaker State, and mostly NAPA oils on this car, and have never had an engine problem with it. I replace the oil every 3,000 miles.