Rubber gasket vs. a cork gasket

When fitting a rubber or cork rocker cover gasket ( pressed steel cover), make sure it's clean and dry ( as is the cyl head too), apply Contact Cement to cover and gasket ( the side that goes to the cover),
and stick it to the cover, wait 20 mins, then fit the cover to the head, dry.
The reason for using Contact Cement is to stop the gasket from sliding/squeezing out under pressure, same as having the cyl head clean and dry too.
Contact Cement isn't affected by oil.