I have a neighbor with.
M6-131 he has had 14 months. Last winter he had a DEF light problem. They came out, drained the DEF (it was the DEF that came with the tractor) and he had no problem - until this winter. Now we are having the coldest period I can remember in ages. It was -42 actual temp yesterday morning for example. Starting when our temperatures have been below zero Fahrenheit he has been having DEF problems again. It has 386 hours. His invoice doesn't have anything special about the DEF written on it but they did change all the fuel and oil filters and oil (his request - Jerry is 91 and doesn't do much of his own maintenance anymore). When you mentioned your DEF problem it sounded so much like Jerry's experience.
I have a
M7-171 I got during this past year. I've had no DEF problems. I used Peak Blue DEF, first having the dealer send the fluid they recommended, later buying it from the local farm store (dealer is 150 miles away). Only thing is my M7 sits out the winter unless we get a real snow where I will need its size. It hasn't run in a day when the morning template been below 20 but the computer on the KVT takes control at cold temperatures. The transmission needs to be warmed up and that warm up time will also give the DEF heater time to melt DEF. I believe my book says if I use DEF faster than the heater can melt it the system will limit power until it can catch up.