I have done natural gas & oil boiler performance calculations for a utility sized unit when I was working for a major boiler vendor. They showed that the efficiency on natural gas was 85.6% vs an oil efficiency of 89.7%, even though the natural gas excess air was 5% vs 10% for oil. Identical...
Just a short note to remember that the efficiencies will be different for the different fuels. The combustion of natural gas produces much more water than oil, so all other things being equal, the efficiency of natural gas will be lower than oil.