Here in the central Vermont we start purchasing winter mixed fuel in early November, I have been told by my dealer that is generally good to approx. Zero degrees +/-, so I also add Power Service. How far South do you need to go before winter mix is no longer available I do not know. I have never had an issue with my old B7100 Kubota, but I added 8-12 oz. of Power Service to the 2.5-3 gallon tank. This winter will be a learning experience with my new L3901, below are some previous learned experiences, the hard way.
In my 04 Dodge Cummins I typically add 8-12 oz. to each fill up, about 20 gallons into a 33 Gal. tank. I have twice experienced gelling of the fuel in my truck. In retrospect I had not add enough PS to compensate for unanticipated large temperature drops. The first time I experienced this issue was in my driveway the engine quit and would not restart, it took 18 hours to get my truck running again after installing a new fuel filter, 911 in the filter and fuel tank along and tarping my truck with a salamander running underneath, the temperature if I recall somewhere near -20+ with a stiff wind. The second time years later on my way home from work at 1:00 am, again the temp was near -20, I pulled off the side of the road, again replaced the fuel filter and filled it with 911 and the remainder in the fuel tank and limped home at 5 MPH for 15 miles. The next morning she fired up and ran like a top.
I put 1 gallon of PS in my Motor Homes 100 gallon fuel tank and have never had an issue, starts all winter. As I mentioned the right concentration in the tank long before any unanticipated temperature drops is probably the key, and make sure you have run the equipment long enough to mix in the complete fuel system and make sure that the water bowl is drained.
As an added insurance I add ½-1 gallon of Kerosene to my truck at each fill up in the dead of winter since it sits Idle for most of the winter now, unless needed for dump trips or plowing on really nasty days, I would not recommend this to anyone especially in a new machine it could create warranty issues since the engine may run a bit hotter and there is less lubricity to the fuel that can affect many components, check with your dealer before attempting this process.
I also add Biobor in my Motor Home since it is apt to set for 3-4 months in the summer without being run enough to use a full tank. I have been told by the local diesel shops it is not need where I live, we do not get enough warm weather to cause fungus in the fuel, but I have heard of some expensive horror stories, it is in my MH tank as cheap insurance.
I Mention these stories because it is one thing to run out of fuel at 70 degrees and I have done it, but quite another at 20 below zero or colder in the middle of your driveway or on the side of the highway, memorable but not fun. And as one of this sites member post states and I will not quote, you cannot live enough to make all the mistakes yourself so learn from others mistakes.