First of all what is DEF? Must have something to do with small truck diesels that I don't know about. I do know about very large marine and industrial diesels. I know what water is and urea contains nitrogen and is used as a fertilizer. I tow with a GMC with a 350, second engine, and it has 304,000 miles on it and tows nicely. No DEF here. No big $$$ here either.