Oil fired heating plant (furnace or boiler system) needs maintenance by a good oil burners service man. There are a lot of service men but only 1 out of 10 Service men are really any good, and of these 1/2 of theses guys are great. With a good burner system and a great service man these heating plants will run great for years. An inexperienced service man will miss the tell tail signs the heating plant needs help. The picture below could tell a heating man what is happening before he even gets to the heating plant.
The picture you are looking at is not mold but instead it’s dirt. In this photo this house has hot water heat. There is no air filtration system in this house, as small dirt particles migrate into the wall mounted radiator (baseboard heat) heats the air as it passes by but the dirt is charged with static electricity than the particles are carried up the wall with the warm air. This is where the charged particle bond cooler parts of the wall. The framing of the house keeps the sheetrock cooler directly in front of the studs in the winter and warmer in the summer. This also happens with electric baseboard heat. Using an air purifier such as a Honeywell 17000 HEPA Quietcare Air Cleaner works great removing all the particulate in the air.