I had a similar issue with my 850, sat overfilled for seven years albeit with only an extra litre or two. No amount of idling/revving would clear the smoke. Once the car was in a state to be driven a good ol italian tuneup did the trick it took about 10 mins of driving to stop smoking although during this time it looked as if someone put oil in the fuel tank. Not sure about having 13 litres, guy musn't have had much going on up there, although I wouldn't see why it would damage the valve stem seals if they're designed to live in oil 24/7. Can't hurt to take it for a spin, it'll cost you nothing.