your engine doesn't know the diff between synthetic and natural oil. U can move back and forth if u wish. The reason some older cars leak with synthetic is that the better oil has a tendancy to clean out the engine. If the engine was bad and leaks were glogged up with crap than synth can clean those spots out and leak. I'm sure ur engine is in better shape than that so u shouldn't have problems. I would go with synth if u don't mind the extra cash.

I use 5w-30 mobil1