The yellow bottle is primarily methanol and used for 4 cycle gasoline engines only. The red is basically isopropyl alchohol and can be used in any type of internal combustion engine like 2 stroke, diesel, and 4 cycle. They both do the same thing although the red bottle (iso-heet) helps clean the carb or injectors. Living up in northern Wisconsin I have cases of this stuff all winter long. I plow with my Brute all winter and use it and treat all vehicles before freeze and all winter long. I run it in just about everything because if their is any moisture or water in any fuel then I can count on having a frozen fuel line or I mostly run into fuel filters full of ice. The weather is already miserable here.