I asked that question of my local Chevron distributor the other day when the temp here had got down to plus 6 F. The answer was that they use additives and not #1 diesel (stove oil) because the #1 has to be added in too large a percentage and is too expensive and lowers the heating value (energy content) of the fuel. She didn't know exactly which additives were used but was able to tell me that they didn't generally worry about it until ambient gets to +5 F.
The additives you might need are probably almost impossible to find in So. Cal even if you knew what to look for.
As you get to Colorado the fuel you buy there will almost certainly be 'winterized' appropriate to the local conditions. Once the car is running, even unwinterized fuel will be OK since the fuel through the fuel return line does a lot to warm the fuel in the tank so the diesel you have in your tank from So Cal will work just fine. If you 'time' your fill ups to buy a nearly full tank of fuel just before you shut it down in Colorado, that new fuel mix should fire the engine up fine the next day.
As always - Buy Clean Fuel; Keep It CLEAN. That instruction from Caterpillar is not a suggestion, it is a COMMAND!