[winkosmosis]

Your engine is MADE for 91/93 octane. By running a lower octane you may not notice a difference but the engine does. It's igniting the fuel earlier to prevent it from pinging, which saps energy because the piston is fighting the burn for longer than it should.

There is a reason the manual says to use 91. If you're so poor you can't afford the extra 20 cents per gallon, you need to sell your Infiniti and buy another car! I suspect you're just being cheap without considering the cost of the wear on the engine.