well I'm running with a M135 prometheus spring, used to run a 18:1 set but had troubles with the gearset i have when fitting them in my gearbox shell, so i had to move to 26:1 (had to since for some reason, those were the only gears that fit and I just happened to have such a bizarre gear ratio set).
Never had any issue with that motor, could say it's pretty robust, since I learned how to open up fully my gun and how to go about with my gearbox, so the motor went through some rough times, but it held up really well!