[WardL]

The gasoline will still be 90 octane in the spring. Gasoline can sit for 3 years or more and be fine if water does not somehow get in. Ethanol is a bad source of gasoline, but it will still burn fine if not exposed to water or high moisture environment (like a boat on water). Octane blends linearly so a 50/50 mix of 87 and 93 makes 90. California only has 91 premium and 90 is pretty close. Since you are guessing, I would wait until spring and drain out a quarter or half of the tank and fill up with 93. Why screw around with your beautiful Roush engine?