I am with BikerBoy and Musky, if the rings are properly bedded down and you only have one oily pipe, the culprit is the sloppy valve guide, or a loose guide when up to operating temperature. That plug should also foul easily, or should be way more sooty then the other plug in the clean pipe side. I have the exact same problem with my 62 RGS, and to test that, hold your finger over the oil pipe return hole in the oil tank, when bike is all up to temp, and force more oil up to rockers at say 1200 rpm, for 30 or 40 seconds, and if the right pipe sends a little smoke out, you have guide problems.