I've been putting half a quart of store bought veg-oil nearly every fill up of my 1984 300TD. It makes the exhaust smell better and the engine has a muted sound. I have no idea about the long term effects on the internal parts of the engine at this level and no one does unless they have disassembled many many engines which employ this amount of veg oil blending. They may tell you these people do this or that- but that advice means little without actual tear down mechanical experience.