“Back when I originally bought a whole-house generator, I discovered that propane isn’t very efficient when powering a generator. I ended up getting both a gasoline powered unit and a diesel powered one to back it up in case the SHTF.”
Also water-cooled diesel engines will run practically forever, water-cooled gasoline engines are next, and then air cooled gasoline, and finally propane - you get limited engine life with propane (as I remember it).
As to efficiency, propane simply has less energy per gallon, roughly 74% of gasoline and 65% of diesel.
You have it backwards. Diesel and gasoline both put particulates in the engine rings, oil, bearings simply by their combustion products and blow by gasses. Propane produces zero particulates. Due to this propane engine oil lasts three to five times longer as there is also zero fuel dilution into the oil propane is a gas it cannot dilute an oil like gasoline or diesel will. The lack of blow by and oil carried particulates into the rings bearings and valve guides means those wear surfaces also last much longer. Propane and natural gas both suffer from a loss of wide open throttle power due to the fact that the fuel gas displaces air in the manifold. This is corrected one of two ways direct injection after the intake valve closes or boosting via a turbo to a higher manifold pressure and therefore charge density. I have a propane / NG fueled generator with over a thousand hours on it it’s only had it’s oil changed twice the interval is every 500 hours after the initial break in period last I pulled the dip stick it was clear like brand new oil even after 500 hours of running it’s still only brown never the ugly black a diesel engine oil would be after a hundred hours.