Rejex. It was made for light aircraft. It repels bug guts and keeps them from sticking. Good stuff, but it is a little spendy and you do have to reapply it every few weeks. But after a good coat of it, you can ride through a swarm of bugs and not have a single one stuck on the windscreen. I've seen them literally bounce off instead of splatting.
Just bought a 12oz bottle on your recommendation. Hope this stuff is as good as they make it out to be.