Gas will mix with gas, there is no need to worry about seperation. Once they are mixed- no worries. The only thing you need to be concerned with is 'double mixing'.. In other words....thinking your gas is 100% race gas and adding pump 91 and unintentionally doing it again, now you have 75% pump 91 and 25% Race gas. Looks can be deceiving even double mixed, it'll still look purple and its hard to tell what the hell it is after its mixed, even when its been double mixed. Don't ask me how I know, I learned the hard and expensive way, cost me 2 pistons and a set of rings-bearings.