"This is a how-to on using veg oil to make your diesel car go, easily, safely and cheaply. It's really very easy, very green and very legal. [...]
The runniest easily-available oil is rapeseed. Next best is a blend of rapeseed and sunflower, which is commonly sold as "vegetable oil". Pure sunflower and corn oils are also good, a little thicker but still perfectly useable. Peanut oil would be perfect, but it's expensive in the UK."