I just cleaned up a couple of remotes for friends of mine. Same situation. a good portion of both boards was coated with a layer of oil.
I don't think it's leeching out of the rubber, unless it's absorbing skin oils like a sponge (which is very well could be).
Regular isopropyl is ok, but 92% is better. If it still doesn't work well, then the little black portion, which is impregnated with conductive material, has likely skinned over with rubber lacking this ingredient. If it's thick enough, you might want to try roughing it up a tiny bit, with some very fine sandpaper. If it is skinned over, this should expose more of the conductive material.
Otherwise, yeah, any thin, conductive material glued onto that area should work until it wears out the material or the traces on the PCB.