Prime Guard 2.5-Gallon Ultrapure Def Diesel Exhaust Fluid. This environmentally friendly solution for Selective Catalytic Reduction (SCR) systems meets the stringent ISO 22241, AUS 32, and DIN 70070 standards for purity and concentration. It is a formaldehyde-free, NOx reducing treatment specifically formulated for diesel engines equipped with SCR systems. API certified for proper SCR operation and meets or exceeds all EPA standards. It includes a nozzle for an easy pour.