Oil to propane, Kingston NY area - EDITED.

Mod cons can give you some indication of their running conditions. They will accumulate running hours and in some cases running hours at various modulation rate. If you have a way to measure or meter the fuel, the number crunching is not so hard. BTU meters are also available, and in some cases billing is done based on BTU meter calculations.

Obviously any properly tuned or maintained boiler should run most efficiently. Mis-sizing and mis-applied boilers are probably more common that not. Or heatinghelp.com would not exist :)

Least we forget the building envelop and occupants use has the most to do with fuel consumption.