Drain and replace the
oil and filter without fail, for no other reason that the fuel will compromise the lubrication qualities of the
oil, and risk damage to the bearings. Its possible that as mentioned fuel may be entering via the fuel pump or what is more common is that the carb is set up rich and unburnt fuel is getting past the rings, check the color of the plugs.
As an example: I'm sure this is not the case with your particular car but once I was witness to a fellow doing a Tune-up on a 3 or 4 year old 74 Torino with a 351 that had been badly neglected as far as routine maintinence goes. This guy was sort of a crusty guy, you know the type: Burley chested, heavy smoker, wearing coveralls in his late 50's or early 60's. Well, while he was working under the hood with the car idling the fuel in the crancase
oil ignihted and literally exploded with enough force to tear the sheet metal valve covers loose from the surrounding bolts that hold them on, same thing for the pan and several of its bolts. It rung this guys bell so hard he left for the rest of the day. You do NOT want to ever experience this.