interesting, I knew a CFI who taught Tailwheel Endorsement and BFR in his experimental airplane (Not a KF). He would collect his normal hourly rate for his instruction and would split the avgas cost with the student. The student would write two separate checks to him, one for CFI services and a second check to cover 50% of fuel cost. I don’t know if it was 100% FAA legal, but he did it for many years. He recently retired at 80+ years of age with 60+ years of flight.