At high mileage it may be best to follow the airheads and treat exhaust valves as consumables - fit new at 60K (I believe) . If true, that gives mine only another 15K :-(
My old R100 used to do that if left on its side stand ; bugger, that thing used to smoke like an Ariel Arrow for the first mile. Problem was oil draining past the rings on the left piston. I used to prop it in its shed on a wooden block under the right cylinder head and I've carried the habit over to my GS