My dad and I once worked out a formula we are convinced would stop the aging process. We couldn't get FDA approval to sell it because the primary ingredient was arsenic.
I took chemistry about 30 years ago so I can't claim to be a current expert. But if my memory is correct, Diesel has about twice the calories gasoline does, and gasoline has a great deal more than propane.
You can get much better mileage burning Diesel than gas, but it comes at a cost. Propane is much cleaner than either the others, but using it also comes at a cost.
I think a 20% increase in fuel economy based on a small amount of added chemical is unlikely. There would either have to be a tremendous amount of energy in that additive or it would need to have some catalytic impact that could be huge.
I can make a 20%+ improvement in fuel economy by adjusting my attitude when I climb behind the wheel. My current daily gets 8 mpg if I have fun and 10 if I drive like an old lady.
I saw gas at $1.89/gallon last week in Portland, and bought a tank at $1.95/gallon. I suspect interest in fuel economy will go down as gas prices have returned to the level we've been used to.
