Had similar issues with oil leaking from the valve cover post-service on my YZF. I actually bought a new valve cover gasket to install. Comparing the old to the new, didn't really see enough of a difference. So I used silicon spray to perfect the old gasket + double/triple check the mating surfaces on the valve cover head. Made sure it was everything was perfectly aligned for installation. Leaks ceased. New gasket stored away for later use.
Stock 2008 Yamaha YZF-R6S Blue >60k miles