Krillitane oil was a substance used by the Krillitanes. Though it was not harmful to other races, and indeed proved to stimulate human intelligence, it was lethal to Krillitanes as they had changed so much from their natural state and would burst into flame if they came into direct contact with it.
The oil’s effects on other races were impressive. It seemed to act as a “lubricating agent” for the human brain, accelerating thought processes and increasing intelligence by a significant factor. The effects of the oil were only temporary. Soon after being ingested, the effects wore off. It would appear that Krillitane oil was not unpalatable, as the method of delivery was frying chips in it. No one who ate the chips, except for the Tenth Doctor, complained about their taste and others, such as Rose Tyler, even complimented the chips’ flavour. (School Reunion)
The oil had become toxic to the Krillitane race, due to the race’s frequent physiological and chemical changes, (School Reunion) some time prior to 1140. (The Krillitane Storm)
Krillitane oil was instrumental to the Krillitanes in their plan to crack the Skasis Paradigm. A group of Krillitanes led by Lassar used its effects to enhance the minds of the entire student body of Deffry Vale High School. The consciousness of the now hyper intelligent children was then collected using Krillitane technology, allowing the students to work in tandem in a way not normally possible.
Several barrels of the oil were blown up by K9, covering the Krillitanes in it and subsequently killing them. (School Reunion)