You use the octane rating that the manufacturer recommends. The octane rating is the propensity of the gas to ignite under compression only. The lower the number, the lower the compression needed to ignite the gas. So the engine compression is measured and the proper octane to use is established based on the highest compression reading. If you use an octane lower than recommended, you run the risk of having the gas in the piston cylinder ignite before the spark, causing a knock and damaging your engine. If you use the same or higher octane, you will have no problems. Using a higher grade does absolutely nothing for you. Your engine will ignite the gas at exactly the same time regardless of octane rating if it's 91 or 93. You will only be paying more for gas than you have to. You will not damage your engine or void your warranty using higher octane, but you possibly might using lower octane.