Zeiton-7

Zeiton-7 is a rare fuel which is used by the TARDIS. It used to generate orbital energy, which allows the TARDIS to move. It can be also used for fuel by other Space and Time ships.

Zeiton-7 is generally found on Varos, though Galatron Mining Corporation found traces of Zeiton-7 on an asteroid. (DW: Vengeance on Varos)