Eurocopter successfully flew a prototype diesel engine last November.
They retrofitted an H120 (5 person helicopter) with the diesel engine and reduced fuel consumption from 220 Pounds Per Hour to 143 PPH. That works out to 21 gallons per hour, which is still a heck of a lot. That might be around 6 MPG, but it is a larger bird.
Would be neat to see what a diesel piston engine in a smaller craft could do, maybe in a coaxial or dual rotor design for fuel efficiency.
I find it hard to believe the Russians are going to get 30 MPG out of their design, and we have yet to see any prototype a year after their announcement.