People freak out when they see the word "silicone" but this is a good place for some. Also Permatex's Right Stuff which hardens more like a good stretchy rubber would be a nice choice. But the trick is to take out the O-ring, run the bead of goo around the groove, replace the O-ring and then put the cover on finger tight only. This squeezes excess goo out from behind the O-ring and allows the O-ring to form to the not so perfect face of the primary cover. Allow goo to harden, remove the cover, clean away excess and replace the cover, this time tightening it down enough to compress the O-ring. The reason you can get away with silicone or Right Stuff in this kind of application is because you are going to clean off the excess and if some where to break free inside the primary, it doesn't block any oiling passage.