How Many Gallons of Oil?

Happy New Year Brownstoners! So 2011 finds me (finally!) the owner of a house in Brooklyn! Yay! As a first time homeowner, I expect my learning curve to be steep and look forward to posting many pleas for advice as we renovate and get the feel for our new place. My first question involves the oil tank. Can anyone give me an idea of how many gallons of oil you burn through a year to heat a 3,000+ square foot brownstone? I’m particularly concerned with the winter months. When we closed this December, the oil tank had 765 gallons in it. We plan on switching to an HVAC system this spring. Do we even need any more oil this winter? Thank you for your help/opinions!