Brushed and vacuumed flue pipe and heat exchanger Replaced oil filter, nozzle, and strainer Checked all electrical connections and safeties Performed combustion and smoke tests 1%o2 14ppm co 0 smoke While performing the maintenance we found very high levels of sludge in the oil filter canister. Also found some rust starting to form in the canister. Recommended adding a sludge additive. After getting customer authorization we added sludge additive to the oil tank.