Valve covers tend to leak at the back or along the bottom (side toward exhaust port) edge. If the oil is on the intake manifold, it may be seeping up around the intake bolts. If this is the case you'll need to remove each one, clean it, and reinstall with thread sealer. Do one bolt at a time.
If you decide it is the valve covers gaskets, be sure to inspect the gasket flange of the covers while they are off to insure that they are flat all the way around. The flanges on stamped steel covers often get dimpled around the bolt holes due to over tightening. An uneven spot in the flange will cause your new gasket to split and leak in the same spot and the old one. If they are uneven, pound them flat with a hammer. Use some lock tight on the bolts to keep them tight.
