Are you sure it's oil and not condensed gasoline-gunk? Remember, the carburetor's at the other end of that hose, and it's cold outside this time of year. If it's actually oil, check inside the distributor (pop the cap) and see if it's oily inside. If so, the distributor is worn and the PCV system may not be functioning properly. Make sure you cannot suck air through the vacuum advance hose when it's connected to the vacuum advance. If you can, replace the advance.