Cork gaskets leak, period. It is a cheap--and old technology. The RTV in a self-releasing can, it is a beautiful thing. It seals and holds---over 2 years now (getting ready to adjust valves again) no leaks. One can does 4 valve cover gaskets easy---I use on motorcycle too. About $15 a can at auto parts store
Permatex Ultra Black Maximum Oil Resistance RTV Silicone Gasket Maker (9.5 oz)