Hi - Oil furnace quit yesterday AM. Thought it was out of fuel since last delivery was November, but we just got a delivery of 150 gals. this morning and it won't start up. I'm suspecting the fuel filter may be clogged with sludge since we let it run out. Anyhow, how hard is it to change the filter? It's outside the furnace, looks like a screw-on thing, just like in a car. And where can I find a replacement? Thanks in advance!