Could be that the seal/gasket might be finished ask the guy if he can drain the oil and take the cover off to take a look at it! If not could be it might just need to be tightened up! Either way it's worth a look I'm it's a beautiful bike there's not too many of them rolling around, especially in that colour!
could just be oil residue from spilling oil when refilling over the years as well. Could go either way IMO.