Oil leak at valve covers

I used to use the black rubber gaskets with a thin coat of RTV on both sides, and making sure I had RTV between the bolt shafts and oil side of the rails.
Never a leak.
Cork always seems to seep eventually, and the rubber with RTV never will.

Now that I have aftermarket heads with machined flat rails I use the metal core rubber gaskets, but still use the RTV exactly the same way.