...ruin the car? Even if only once? I have only done it once, and i added about 7 gallons of it. I believe there was about a little less then a quarter tank of premium left in it.

The only thing the octane does is keep the upper engine from chattering or pinging.

If you filled up with just normal over premium time after time. Then you might cause damage only in the form of lower horsepower, wasted gas and carbon build up. But to actually "harm" it. No.

The only thing the octane does is keep the upper engine from chattering or pinging.

If you filled up with just normal over premium time after time. Then you might cause damage only in the form of lower horsepower, wasted gas and carbon build up. But to actually "harm" it. No.