All fuel oil has a wax in it, and kerosene does not. Fuel oil congeals when the temperature drops low enough so that the wax begins to separate out into small specs. When enough of these specs join together, they can form a clog. We strongly recommend using kerosene in your outside tank year round. If you choose to use fuel oil please understand that it may cause you to have a system shut down. This would cause a no heat call and could damage your unit, or to your home if it freezes. It most cases it is cheaper to use kerosene, than to repair your unit or home.