I have 1000 gallons of "summer" diesel here and use it all winter. Usually in temps from 0 to 30F in the dead of winter. I add some Power Service anti-gel to it and run at temps to below 0 easily. If it's 20 below (or colder) F I don't want to run any diesels anyway.
Winter diesel is less efficient. Dollar for BTU its more cost effective to buy summer diesel and treat it.