I agree that this adage is mostly urban myth, but wouldn't one get condensation on the rare case where you filled up on a warm, humid day, then the weather turns bitter cold, as it's happened here recently. During the last warm spell we got the dew point went up near 60. Everything in the garage was wet... the floor, my car, my tools... everything that was chilled. Should that humid air have gotten into the tank, I'm sure it would have condensed now that temps are back in the 30s.

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95 E320 Cabriolet, 159K