Veg oil
There's a foreign web site that lists all common diesel engines and tells you how much oil you can add to an undapted engine i.e. my 2.2tdi Frontera and my old 2.5 mazda bongo were both good for 30% 'cooking' oil, the Bongo only has a two-way particulate trap and smelt like a chippy on anything over 10%!! The Frontera has a 3-way cat and has NO smell on up to 30%!!
There are many conversions and heaters available, and methods for removing the thick Glycerine content.
My new motorhome has a 2.8jdi unit with high pressure injectors which are unsuitable... thats progress for you!!