idk if you are supposed to, but the reason for 91, 93 ect octane gas, is because when your piston moves up, the compression can explode the gas itself, without spark. this is a pre detonation, also known as knocking. you can hear it if it is happening, if it is, you need to drain your gas and move up to a higher octane, knocking is horribly bad on your engine, “most” factory engines dont build enough compression to worry about this, however, when i threw in my weisco high comp piston in my dirt bike, i had to move up to 93 octane. so, i would just try a gallon or so of 89, and if your car starts to knock, drain it, and go get some 91 or whatever.