Hmm, I am now too ancient to fly anymore, but I just wonder how some people with correlators, electric fuel shutoff valves, hydraulic controls etc.,
would have managed with older piston machines that had none of them!
Cannot remember which way the H300 engine is mounted, but I do remember that my old Hiller was vertical. As an engineer I cannot agree about "friction" being the cause of a difference in ROD. But there will certainly be some differences in forces because of the gyro effect of an engine between running or stopped. May not be the cause, but it should be considered.