veg oil, i have run it through many different tired diesel engines and ALWAYS see a significant step-up in performance. It dissolves the diesel residue that alters the injector spray pattern. no good for common rail engines though.
Chuck in half a tank of veg to half a tank of diesel, any more veg oil and you will need to thin it out with petrol or kerosine.
Regular use may perish rubber in the fuel line, if you have any, other than that no downside I am aware of. Used to be a cost saver, but big bro has cottoned on and veg oil is now as expensive as diesel. EXHAUST WILL SMELL LIKE DUNKIN' DONUTS FOR A BIT !!!