Don't do it!!
Do not alternate fuels.The computer learns what octane you are feeding it,over several tankfulls, and will make the car respond like it is being fed.
More octane, better performance,less octane, less performance.
Also detergent packages, and additives are greater in premium fuels, and with only a 13.2 gallon tank, it does not cost much to get the best.
Most of all, if you alternate, the car will be unsure what you want, or what it should do, it will be constantly guessing what you are going to give it.Like your kids, consistency is the best thing.