Shop for DEF Fluid Diesel exhaust fluid DEF, 55 Gal. Drum Barrel. Compare Prices from Walmart, Napa, Autoparts, Oreilly, Amazon, Northern tool, Grainger, bulk DEF Diesel Fluid (DEF) is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. It is standardized as AUS 32 (aqueous urea solution) in ISO 22241.