Receipt for the gas, receipt for the tow, receipt for the repairs, and a letter from the mechanic.

That is how I would approach it anyway.

But my question is how could the mechanic prove that it was bad gas that caused the mechanical problems and not some unexpected engine trouble? I'm sure you know a helluva lot more about it. How could you prove that it was their gas and not whatever was left in the tank from another gas station. Seems like way too many variables to legally prove it was QT's fault. For Joe's sake, I hope I'm wrong...

I had a buddy who got a new fuel system in a truck from QT after it went bad on him. Granted he was a regular QT customer and could show he always bought his gas from them for his lawn care business. QT did back up their guarantee for him though. It's worth a shot, especially if your mechanic says that's the problem.