If the manufacturer calls for 91+ octane, you must use the higher octane rating to minimize engine knock.The reason for the higher octane numbers is to reduce what is known as spontaneous ignition. For a car to run optimally, a flame front needs to be in the proper direction. Typically the flame front is in a radial direction away from the spark plug.However, with high compression engines (to get more power), gasoline in the cylinder can reach temperatures that will cause spontaneous ignition in various places inside the cylinder. These other flame fronts will collide with the flame front coming from the spark plug, and usually results in what is commonly known as Engine knock. This is the sound you hear then the flame fronts colliding with each other.The shock waves generated WILL cause engine damage and excessive wear over time.If you can afford a new 2006 Acura TSX, then the gasoline cost should NOT be a factor. Considering that 91 octane is typically only $.20-$.30 more per gallon, in a car like this, it will only amount to $3-$4 to fill up your tank as opposed to the lower octane gasoline. If it's the car you want, this additional $3 cost should not be stopping you. Even if you fill up once a week, you're only looking at $12 more a month?Get the car, put the good gasoline in it. Lifes to short to be cheap.OK, went to Acura site, it says recommended 91+ octane, performance lower when using lower octane. So this vehicle has a knock sensor to delay when the spark plug ignites ( retard spark). You can put what ever gas you want in it, it will just reduce performance but not harm the engine