If you think about it, its a renewable source of fuel.
If there was any waste from using oxygen as a fuel it would carbon dioxide.
The aliens could dump the waste into the atmosphere and then fill up knowing that the carbon dioxide will be used by the plants to produce more oxygen!

Pure speculation, not even a hint of evidence of it that I can recall...

And, as Byrd pointed out, it isn't exactly rare....

Oxygen is an element though, I didnt think you could create the elements. Well I know that we cant :S

Actually, we have synthesized various elements, and we do create different isotopes, hehe... We have also broken apart molecules to obtain elements...

Besides, the element they seem to use isn't found here (Uninpentium right? hehe.....)

[edit on 19-7-2004 by Gazrok]