I used a thin layer of high temp silicon on both sides of my rocker cover gasket as, it wouldn't seal. I also thinly coated the spark plug sealing 'doughnuts'.
That was a brand new gasket too !
Of course, having said that, I didn't use any sealant on the gasket the first time I fitted it so, the oil seapage could have been from the areas Mr Weasel mentioned ?!