What is the best product for sealing the galvanized pipe from the oil furnace into the chimney flue? Is basic mortar okay, or should I use a furnace cement or some other product?
High temp furnace cement is a good call. I would first recommend an NFPA-211 level II inspection. Feel free to use the links at the bottom of my home page at www.brooks-hearth.com to help find an inspector offering these inspections. Good luck.