I'm not sure how many people are familiar with the term "sour gas" but it refers to gas that gets too old, and basically as I understand it, old gas builds up peroxide (I'm not sure by what chemical mechanism) which can damage elastomeric engine parts, leading to corrosion.

Today someone asked me when the last time I filled my tank was. I had no idea. I have about 12,300 miles on my HCH, which I bought at the end of March of 2004. This means it takes me approximately 1 month to go through a tank of gas. Does anyone know how long it takes for gas to "turn" ? I have no idea how long is too long, and wasn't successful in google-ing the answer. I also apologize if this question's already been answered here before. I've started to think maybe I should only fill up half-way to keep my car lighter and prevent my gas from turning bad.

My next question is even tougher: does anyone know if ethanol and biodiesel also "sour" ?