ok first your lhs lied to get you to spend more money. you shouldnt have to change fuel for colder weather. and going from 20 - 15, is not a good idea. i was always told that it will mess up the engine and not run right if you go down, unless you rebuild the engine. and when you say it went bad, is this because it made your truck run bad or it just changed color? the color in nitro fuel is just simply dye, and is only there so you can see how full your tank is.