Not sure if you are still looking for an answer, but if you are hopefully this will help. Oil furnaces have to be serviced every year to insure their proper and safe operation. The flue tubes need to be cleaned to keep from a backdraft into the burner assembly. The fouling that you are seeing is an indication that you could be close to a total closure of the flue tube in the heat exchanger. The best thing to do to insure yourself against premature failure and safe operation, is to have a professional come out and do the work. If you are interested in learning how to do the maintenance and assume the liability for your self, ask the technician if he would be willing to show you the needed proceedure. Keep him in mind for parts if he is willing to do so, just remember he made the initial investment in himself and this would be a favor to you if he chooses to do so.
Character is doing the right thing even when no one is looking. Intelligence is understanding and practicing the meaning of character.