I saw the PF farm video about a year ago and with his results I figured it was at the worst not snake oil and at the best will cut down on engine wear in my older cars.
I added MK to my '89 Geo Metro both in transaxle and crankcase. 6000 miles later I cannot testify that it improved mileage since I have made other changes to the car along the way, but my average mpg has increased and my trans noises in reverse have subsided
Is this because of MK? IDK, but since I substituted the recommended 5w30 for a thinner 5w20, I worry less about frictional damage under hotter temps and heavier loads.
Next oil change I might even go to 0W20