9 out of 10 times, when you change fuels, you have to tune. Mixing fuels would only make matters worse. Besides, it doesn't take very long to burn a tank of fuel. Just finish it off before you switch. I've also heard horror stories about bearings going to hell right after a fuel swap. It's really best to pick the quality fuel that is most abundant in your area and stick to it. Byron's has worked great for me.