Very interesting , 6 years flying chieftains and Navajos and company transitioned from ROP exactly as stated above to LOP towards the end of my time there . We had engines running well over their normal OH times by going on condition inspections . LOP was not popular at the beginning by us but after reading posts here it became a acceptable operating technique. I finished up with the aircraft just prior to the vortex generators arriving , just a happy coincidence in my mind . Good memories of helping in hangar and trying to get those 2 red lights to buzz and light up at same time .