Don't know the energy equivalent between propane and fuel oil, but I'm sure you can find it with a search. then, you can compare the cost per BTU. A gas-fired boiler is much easier to modulate, and that may make it much more efficient than a fixed output oil burner. This would let the boiler match the exact heat need fo the building, and not cycle as much. But, it will cost more in startup, since you'll need the tank and a modulating boiler is more expensive than a fixed output version. Many use direct vent and are closed combustion, so this helps in efficiency as well since it isn't drawing combustion air through the cracks in the house (there's less to reheat, and the house would feel less drafty). Some utility companies are offering rebates for efficiency (especially if you had natural gas, but maybe still for propane). The tax credit has expired (I think). So, lots of things to investigate. A 30+ year old boiler is almost certainly much oversized unless you've added on lots to the building, so first thing is a good analysis of the building. You can get an idea by comparing oil use to the heating degree day values for your location. You'll probably be surprised at how low that is. For efficiency, the worst thing is an oversized boiler!