As long as the engine isn't using or loosing oil now, you shouldn't have any problem going to a bit lighter viscosity. You might pick up a bit of fuel economy or pep, since the lighter oil will have just a bit less internal drag.

Same goes for a change to synthetic, if not using it now. If you have reasonably high mileage, you should probably use a good engine flush before switching to synthetic. I recommend the flush made by AMSOIL or BG. If you are willing to take longer to flush, you might want to try Auto-Rx.