i'm running used motor oil mixed with diesel, i run it at a 50/50 mix in my 6.9, i bought the truck just to experiment with for a few hundred bucks, i've already saved more than that in fuel cost, i about at 8000 miles with no problems, i'll probably try to switch to used veggy when the weather gets better, if you start mixing fuel i would start off at something like 25% used then work your way up, there are alot of horror stories on here about guys tearing up there motors but there are alot of guys who are having no problems at all, there just not saying much because they've got nothing to bitch about, if i tried something and it didn't work and i thought it was because of what i did i would probably talk bad about what i did to, but i doubt that most of the guys with problems can actually say 100% without a doubt that bad things happened because of the oil, things break every day in trucks that have never had anything but pure fuel run thru them, good stories don't make news show, bad stories do