More nitro makes the engine start and run better in colder weather. However 10% nitro is good enough for colder weather. I have been flying a lot in the -10, -18 degree celsius range. My Saito fourstrokes and OS twostrokes will run and idle great and very low. I also use OS-F glow plugs in all the two and fourstrokes. You just need to set the needles slightly richer than in hot summer days.