Krillitane oil

Krillitane oil is a substance either secreted or manufactured by the Krillitane race.
 * Evidence suggests the latter, as the Doctor states that they can 'no longer' tolerate their 'own' oil, suggesting that they once could and did often.

The oil is now toxic to the Krillitane race, due to the race's frequent physiological and chemical changes. Its effects on other races are apparently unique, as it seems to act as a 'lubricating agent' for the Human brain, accelerating thought processes and seemingly increasing intelligence by a significant factor.

Krillitane oil was instrumental to the Krillitane in their plan to crack the Skasis Paradigm, using its effects to enhance the minds of an entire school, and then harnessing the collective consciousness of the students to work in tandem. The effects of the oil would prove to be temporary.
 * Rose Tyler (who had ingested some of the oil previously in the adventure) would lose her own enhanced computational abilities over time.

It would appear that Krillitane oil is at least palatable, as the primary method of delivery was frying chips (french fries) in it. (DW: "School Reunion")