UFO - what does it stand for?

A flying saucer was low on fuel, so it landed by a gas station on a
lonely country road.

On its side were the letters "UFO." The gas station attendant was
stunned, but his curiosity got the best of him. "Does that stand for
Unidentified Flying Object?" he asked.

"No," one of the other-worldly travelers responded, "It stands for
"Unleaded Fuel Only."