Performed a heating pm on (2) oil furnaces and (1) oil water heater. Brushed and vacuumed. Replaced the oil filter, nozzles and strainers. Checked the air filters - they still look ok. Started and checked the safeties. Performed a combustion test. System is operating properly. While replacing the oil filter, notice the filter was dirty and had a few drops of water in the bottom of the canister. Recommend adding an additive to the tank to help keep the oil clean. Added a bottle of Hot Shot oil additive to the tank. The water heater is 10 yrs old. Recommend replacing the anode rods in the tank.