I'm not fully edjumakated about biodiesel, but from what I've read, yes, you are hurting your truck. Using straight vegggie oil especially used is putting soaps and water and probably little bits of french fries through your engine. To run straight veggie there are mods you have to do to your engine to enable it to handle all the extra goodies. Even then your engine's not going to last as long as it should. Do a Google search on bio diesel. There is a plethora of sites that will tell you what's involved.