valve covers that won't leak - ??

wow...everybody has a different answer.I have never had a cork gasket leak but i have had the more expensive rubber ones "seep" The problem is over tightening and distorting the valve cover.Cork is best if your covers are straight and you drive regularly.The cork needs to be kept wet to seal.Think of a wine cork..that's why they are stored on their side..to keep the cork wet and swollen.