Is a propane supply necessary for a standby generator?
Let's look at your loads.

Your furnace loads are fairly minimal; you can look at the nameplate data but water is easy to circulate. 600 watts wouldn't surprise me.
Your refrigerator requires about 120 watts when it is running. Overall it needs about 1 kilowatt-hour per day.
Your freezer requires about 120 watts when running.  Overall it uses about 0.7 KWH per day.

So far that's only about 850 watts... won't even make a small Honda generator sweat.

You want a well pump to operate.  Most well pumps are &quot;hard start&quot; meaning they presume an unlimited amount of power available from the utility.  As such, they need a large generator to start.  So you are probably at a 6000W generator just to be able to pick up the well pump without tripping out.   (It's a shame; many wells go in with 3-wire+ground wiring because they put the start capacitor at the top of the well. Those same wires could run 3-phase delta driven from a VFD up top, so soft-start would be easy).

Your electric range requires as much as 6000 watts when it is running.


Holy smoke!  Now we need a generator 8 times larger, +++.  Do you really need to cook Thanksgiving dinner during a power outage?
Yesterday's tech
Psst: Furnace oil is close enough to diesel that an older diesel generator won't care.  The newest diesel generators may have emission controls that will not work with the sulfur in furnace oil.   Do not put it in your diesel car unless you like very hefty fines for evasion of road tax.
The good news for your range is diesel generators tend to be fairly hefty, so no problem with Thanksgiving dinner.
Also, they often tend to be water cooled, and that's free heat you can use!
By the way, fred_dot_u hit on an important limitation of propane.  Propane is stored as liquid, and it must boil to deliver to your generator in gaseous form.  Boiling requires the latent heat of vaporization to be absorbed.  This is why propane bottles get colder when they are in use, and even &quot;frost up&quot;.  Look at how the air conditioning cycle works, and you will realize what's needing to happen inside the propane tank.  It's a serious complication during cold snaps.
Tomorrow's tech
Given your small loads other than cooking, a battery system may suffice.   The fridge and freezer need less than 2 KWH per day.  Your furnace if it runs 33% of the time (8 hours out of 24) needs - well, look at its nameplate but 600W x 8 hours = 4800WH or 4.8 KWH per day.
Your well is all about the surge current; once it's running it is more reasonable. And it only runs when a spigot is on.
A big lead-acid battery is about 1 KWH ($150).  A used Tesla Model S module is 6 KWH ($1100).
Size your battery charger so it can take in the full output of a little gas Honda generator such as an Eu2000.  That means the battery doesn't need to make it through the entire outage: you can fire up the Honda at a time of your choosing to top up the batteries.
You want a fairly large, say 4000 watt inverter anyway, so it can handle the startup surge of the refrigerator and freezer (which will sometimes start at the same time).  So it isn't a huge stretch to bump it up to a 6000W unit to handle the well also.  At that point you have some headroom to run the stove under.
If you crunch the numbers, you can see that batteries, inverter, and battery charger are costly, but not as costly as either a diesel generator, or a propane generator + propane tank provisioning.
It simply becomes a matter of your choices of how much you choose to run the various utilities.  If you hunker down in survival mode and do minimal cooking and water running, you won't need as much battery as if  you decide to have long showers and cook Thanksgiving dinner.