I wonder how using gas that's harder to ignite somehow makes the generator more efficient?
If I'm not mistaken, it's got nothing to do with how hard it is to ignite. More to do with how much energy is contained in a unit of fuel. Propane has more energy per gallon than NG, and Gasoline has more energy per gallon than propane.
Also if I'm not mistaken, diesel is more efficient than gasoline.