they all do that after a while.....try replacing the oil seals that are underneath the bolts (if you haven't already).
sims & rohm (back when they were sims & rohm
) used to make replacement valve cover bolts for these bikes, that had integrated o-rings into them to prevent the leaking.
if all else fails, a bit of silicone rtv should put an end to the leaking, but it's really more of an annoyance than a danger. same goes for the oil line that runs up from the lower crankcase to the valve cover....it tends to start seeping at the joints.