have both os 91 and older rossi.81 on byron fan. the rossi runs with no tinkering, its on an enforcer and fly it 3-4 times each day it has gone out. now in th 40+ flights. the os in a dynmx set up. I would think it probibly comes from the availabilty of os and its parts. but even I would lean towards os for those reasons when i buy new and dont have to squeez the most out of a fan( heavy jet)