Ther are many components to efficiency, but the main two are...
* Exhaust temperature. lower is better.
* Control of excess oxygen, or draft control.
The fuel type is not specifically relevant, but generally proper draft control and low stack temperatures (high efficiency) are simpler to achieve with propane. That said, diesel clearly offers the best combination of simple and compact "BTU storage" in a fuel tank and is more efficient in that sense.
I suggest Googling general furnace efficiency to learn more.
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")
"Well, I just climb up to them."
by Joe Brown, English rock climber
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