Actually the national oil heat council had verified the costs per unit and nobody disputes the fact that oil has beaten gas, 12 years over the last 15 years in terms of unit costs.
Since energy usage (any fuel) is measured in unit value...Oil is pretty consistent in being a better value.
Also, the latent heat gain over gas across the same casting is additionally 10%greater in most all cases....sometimes as much as 15%.
Where oil gets beaten is typically in the maintenance of that heating equipment or design of that equipment chosen.
If you are referring to most U.S. manufactured burners...you are going to get beat by the sooting that occurs by either basic set up or simple simplistic design which aids in this sooting.
Updraft castings add to this dilemma and when combining any updraft with any non-positive pressure burner....gas will do better due simply to the effect across the casting.
That being said....a scotch marine casting with a positive pressure burner (most) will out perform a comparative gas match.
I give money back guarantees on my conversions and (as of today) I have never had to offer a refund when recalculation the degree day to find gains/losses over any of my gas/oil conversions.
Generally, I find that people who are backing gas do so because they cant/wont build/tune a proper oil appliance.