Performed a heating pm on an oil boiler. Brushed and vacuumed. Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. System is operating properly. The oil leak at the tank was from the bottom of the oil filter canister. The inside was pitted and staring to leak. Replaced the bottom half of the oil filter canister and checked for leaks. There is rust on top of the extrol tank from a previous leak. Recommend replacing before it starts to leak.