Odds are, you can add a tiny drop of fuel additive, or cat pee, or moose vomit, or sulfuric acid, or even liquid nitro to your fuel tank with no harm, because it will be diluted by the gasoline. Yes, the additive WILL increase mileage, because it extends the amount of gasoline you have. Check the label, there should be a certain amount you can add per gallon of fuel, if you have a gas can, add it to the can in the proper ratio. I have always been skeptical of fuel additives myself, calling them "mouse milk"....