Hi Rich!
If it is the rocker gaskets, i only use the cork gaskets, i'd only ever thought thats all you could get anyway, didn't know there were rubber ones available.
Having just fitted new alloy finned rocker covers to mine, i never use any silicon sealer, as if any comes off inside the cover, it may get into a oil way, i hate the stuff! I run a good healthy smear of grease around the cover lip right to the edge, maybe half a mill or so thick, then put on the cork gasket, and repeat with the grease. Then fit. Never had a leak yet! Well, not from the rocker covers anyways!
Maybe your cover is distorted if it's leaking from there. I have some cork gaskets, and you can have my old covers if you like, they were not leaking before i changed them. Normally if the leak, they tend to get on the heat exchanger and stink.
I still have a little leak on my engine somewhere, there is certainly drips on my sump plus/strainer area, i think i need a new strainer plate etc as mentioned in my earlier post, may look out for a thicker billet/stainless one?
Cheers!
Alistair