No idea it’s been in the news lately, probably fertilizer related if I had to guess. Here’s my take FWIW:

DEF is part of Diesel engine emissions requirements that ruins the engine and makes it less economical, basically the opposite of what they claim to want to accomplish. Started with egr in 2005 then ultra low sulfur diesel in 2007. DEF fluid came shortly after. DEF is basically horse piss that is injected into the exhaust. The engine cycles at a high rpm for upwards of an hour or more (bad fuel economy) while injecting said horse piss into the exhaust. The ultra low sulfur doesn’t have lubricating properties that the old “red diesel” had and has been known to shred fuel pumps. Run out of fluid, guess what? Vehicle goes into “limp mode” and won’t run over 35mph, or shutdown entirely in some cases.

Deletion (highly illegal) of emissions equipment is not only possible, but actually yields an increase of mpg upwards of 30%. It’s entirely possible, and more cost effective, to do this at the factory but they don’t. Proper tuning will get you more mpg, horsepower, and torque. I’ve done this myself and seen the increases stated.

Item number 117 on my wish list after the dust settles is to do away with this idiotic requirement.