Do I need to run 91 or higher with every fill-up? Can I run 87 or E15(Ethonel/gas mix(88)), and put 93 in every fourth tank? This is my commuter car and it sits in a lot of Chicago traffic. Of course, since people are going back to work the prices seem to be rising again at almost $3 a gallon for 93 octane. Thanks for any input.