Sorry, but running your rig on veggie oil would render you the biggest tight-arse I can think of...even more than the Mazda 121 owner with LPG.
Just put in the fluids it's designed for and harden up about prices...diesel is cheaper than petrol afterall. If running a 2.5lt diesel (or whatever) is too much, then buy a hatchback for gawd-sakes and adjust the lifestyle. I understand that fuel of any type is more expensive than here, but! Sure you're being good to the environment n all, n good on you, but honestly...