Up here in the frozen tundra.... they switch over to winterized fuel at a certain date. The summer fuel that is already in underground tanks at the stations is 'usually' treated in place by well run stations/distributors. Just like in the Spring, they sell the old Winter diesel as they add new Summer diesel to their tanks. They don't pump out all the summer fuel and replace everything with winter fuel. There also X amount of stations that probably skip this step to save money, and distributors too. Adding a supplement may not be needed, but is somewhat of an insurance that the fuel you are putting in your steed is winterized. You are not obligated to add any winter supplement like Power Service to the fuel you use... just some of us sleep better at night knowing it's there. Also, maybe some folks have had to go thru the plugged fuel filter/tank screen/etc. problem, and found how much fun it is to deal with something like this when it's cold and windy outside... and learned better.
