of course it says u should use it but i doesn't say it will hurt ur car if u dont. Some cars can handle lower octane and some cant that y i asked.

 

mine says "premium unleaded fuel only" on my filler door, so thats what i use

 

It depends on the timing and retard. If your car runs without knocking on a lower octane, it can be used - it's not better, but it can be used.

 

I'm lucky to get 14-15mpg around town - but get perhaps mid 20s on the open road (under triple digits...well most of the time) - this is an Avant with APR chip, HKS intake, Baily DVs, Apexi exhaust...just installed a GIAC X chip (seems very nice) - don't know what, if any, mPG hit I might take.