As a truck driver that drives coast to coast, and southern board to northern border in a semi-truck ... I treat my fuel if I'm going to be operating below +30°F ... The "cloud point" of untreated diesel can be +32°F ... If buying in the South and I have a full 190 gallons on board, headed north, I treat it before getting to the cold ... My company recommends treating at freezing (+32°F) we usually end up buying B5-B20 biodiesel which tends to have a higher cloud point, and a slightly higher chance of water/condensation in it ... We buy from major truck stops only. Pilot/Flying J, or Love's. (I do see the PFJ's up north generally have a treatment bottles where there tank is in the winter, but don't know how much is treated, so I still treat it!)
The specific challenges posed by the variability in diesel fuel quality as it relates to winter operability in North America
powerservice.com
I too like and use the HotShot winter defense in my semi-truck (company buys it!) I use there "Everyday" in the summer for my tractors, along with a half dose of there LX4 for the added lubrication, as the ULSD has lost alot of the lubricity diesel use to have ... But this next fill will be putting in the winter defense in place of the everyday, both have a boost of centane too.
I use ORD off-road diesel dyed red, no biodiesel! From a local farm supply, The local county uses this supplier for both off road and there on road diesel too for school buses, road truck, ambulance and fire trucks ...
Check your fuel filter housing, some have a drain petcock you can use to drain off water, the water is clear and sits on the bottom, and will freeze when cold ...
I use 5W40 synthetic motor oil, all year long ...