I've got a 10 year old oil furnace in the basement of a ranch house. The combustion air intake is via the ambient basement air. Is is possible to take the air from the outside (unheated) into the combustion chamber (assuming vents exist to do this on the combustion chamber (or oil burner))? The goal is to keep the basement warmer by not replacing the air exhausted up the chimney with the cold outside air. Any potential downsides to this ?

(I'm sure this has been covered before, but I can't find it)

Thanks in advance.