You should google first. Furnace oil is a blend, but typical Calorific value is around 10,000 cal/g. Density varies but it is easy to weigh a volume for yourself.
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All that depends on where you are at. There are various different grades and those values change from each one. Where someone in a very cold climate may use No. 3 fuel oil someone in a moderate climate may use No 6. The furnace manufacture recommends which one to use.
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