Paraffin oil when used as a fuel for jet aircraft (E, not I) (8)
I believe the answer is:
kerosene
(Other definitions for kerosene that I've seen before include "Combustible" , "Sore knee (anag.)" , "Oil used as fuel in lamps and heaters" , "Paraffin oil (E, not I)" , "No seeker for American fuel oil" .)