I've had this problem since I first got my Vulcan 900. It doesn't leak much, just a little, but enough that I had to wipe it with a rag almost every time I rode the bike. Now it no longer leaks. And I understand this bit of oil came from the crankcase breather into the air filter. I corrected this by ordering the 2 o-ring gaskets on either side of the air filter, at my local Kawasaki dealer. The leaking gasket was the one behind the air filter. When you buy a new air filter from the dealer, it comes with the front one.Total cost about $11.00 for both. The old o-ring gaskets fairly quickly get shaped into square cross section instead of round, so watch out for this if you decide to replace them.