What are the real fuel requirements. All guesses, hints, crystal ball suggestions ne
 

2006, RX8, 118,000 miles, car runs great, car starts instantly hot or cold, plenty of power. I have been using mid-grade 81 octane with no knocking. Fuel prices just went crazy. Mid-grade just jumped to$2.81, regular grade $2.48 and premium 93 octane is now 3.21. All fuels are from Chevron with ethonal, the is a nearby Marathon with pure no ethonal added for about 50 cents more per gallon. Engine has never knocked with any fuel I have used. Car is not raced, on occasion when temps are up and normal I wind it out to 9,000 RPM through third gear. By using lower octane fuels am I damaging the engine? I try to never lug the engine.
Thanks for your time all help, GUESSES, ideas, crystal ball suggestions gratefully accepted