Bryce,
Diesel fuel #2 (DF-2) is standard diesel fuel. It consists of hydrocarbons ranging from C14 to C20. The 90% distillation point has a max. spec. of 640 F. The viscosity at 104 F must be in the range of 1.9 to 4.1 mm2/s. The lubricity spec. is a max. wear scar of 520 microns on the high frequency reciprocating rig (HFRR). (pg. 46 of
https://www.chevron.com/-/media/chevron/operations/documents/diesel-fuel-tech-review.pdf)
Diesel fuel #1 (DF-1) is winter grade diesel fuel. It consists of hydrocarbons ranging from C10 to C16. The cut is essentially a kerosene, and is the same cut as used for Jet-A commercial aviation fuel, and for JP-8 which is similar to Jet-A but with the addition of corrosion inhibitors and anit-icing additives. The 90% distillation point has a max. spec. of 550 F. The viscosity at 104 F must be in the range of 1.3 to 2.4 mm2/s. The lubricity spec. is a max. wear scar of 520 microns on the high frequency reciprocating rig (HFRR). (Same ref. as above.)
So the two fuels have the same lubricity spec., but indeed there is more worry about lubricity for DF-1 than for DF-2. DF-2 is more viscous, has a higher boiling range (less volatile), and typically a significantly higher waxing temperature.
In order to get a single fuel to the front line, the military wanted to standardize on using JP-8 (a kerosene fuel) as its single fuel, replacing JP-4 (gasoline type fuel for jet engines), diesel fuel (DF-2) for its trucks and Humvees, and gasoline for its light-duty vehicles. Extensive tests were performed with JP-8 in diesel engines in support of this effort. They found that there were some lubricity problems (reduced life) in rotary distributor injection pumps used in the GM diesels in the Humvees. However, these tests were not at low temps. where DF-1 would normally be used, but at high temps. where the viscosity of JP-8 is low.
The Kubota diesel engines use in-line fuel injection pumps, and they are less sensitive to lubricity than are the rotary distributor pumps.
My opinion is that it should be safe to use kerosene or DF-1 in the Kubota engine full strength at low temps., but it might be safer to make a 50%/50% blend of DF-1 (kerosene) / DF-2, and/or use a lubricity additive to help cover for potential shortcomings in the lubricity of the kerosene.
DF-1 and kerosene are similar in terms of the base fuel, but DF-1 has a lubricity spec., while kerosene does not. DF-1 may contain additives to meet its specs., while kerosene would not require those additives. I will use a lubricity additive with any of the fuels I use, and that additive also includes a cetane improver, a corrosion inhibitor, an anti-gel compound, and a deposit controller. This approach has some slight downside of possibly reacting with other additives that might already be in the fuel as delivered.
In Alaska the old pilots flying trap lines would take the oil out of their planes put it by the wood stove in the cabin at night then put it back in the motor in the morning!