I don't have any direct experience using DEF on my older Powerstrokes but here are some facts about DEF that might help you make a decision:
Q. What is Diesel Exhaust Fluid (DEF)?
A. DEF is the reactant necessary for the functionality of the SCR system. It is a carefully blended
aqueous urea solution of 32.5% high purity urea and 67.5% deionized water.
Q. What is the freeze point of DEF?
A. A 32.5% solution of DEF will begin to crystallize and freeze at 12 deg F (-11 deg C). At 32.5%, both
the urea and water will freeze at the same rate, ensuring that as it thaws, the fluid does not become
diluted, or over concentrated. The freezing and unthawing of DEF will not cause degradation of the
Q. How do I keep the DEF from freezing? What happens if the DEF freezes
in the tank on the vehicle?
A. During vehicle operation, SCR systems are designed to provide heating for the DEF tank and supply
lines. If DEF freezes when the vehicle is shut down, start up and normal operation of the vehicle will not
be inhibited. The SCR heating system is designed to quickly return the DEF to liquid form and the
operation of the vehicle will not be impacted. The freezing and unthawing of DEF will not cause
degradation of the product.
Q. Can an anti-gelling or freeze point improver be added to the DEF to
prevent it from freezing?
A. No. While an additive could improve freeze point of the mixture, the 32.5% solution is very specific to
providing NOx reducing properties. Any further blending or adjusting of the DEF mixture will impede its
ability to perform correctly and may cause damage to the SCR components. Additives of any type are not
approved for use in DEF today. If the ISO standards should change to allow antifreeze additives,
Cummins Filtration will ensure our product continues to meet ISO requirements.
Taken from a Cummins website: