Paul,
You'll love Justin's silicon gasket. I've had it on my car for at least 5 years now and there has never been a single drop of oil get past it. It is truly leak proof. The best part is that you can take the valve cover off as many times as you want without ever replacing the gasket. I've had mine off 1/2 dozen times, as recently as last week, and it just seals back up with no leaks.