I have no problem using 'old' diesel fuel. I just salvaged 160 gal that was
6-8 years old and it is clean and clear. Most likely my fuel had some
biocide put in it to retard the growth of bacteria in the bottom of the tank
(if water is present).

In your case, I'm not sure what has cause this fuel to be reduced to syrup.
I have seen this happen with really old gasoline..

The simplest thing, would be to mix it with enough heating fuel and burn it
in your heater. You might do this as part of the tank rinsing process since
clean diesel fuel should clean up the tank.

My local automotive/truck mechanic heats his shop with a mixture of fuel oil
and old crankcase oil.

I recommend you burn it rather than trying to dispose of it since most
disposal outfits will charge you more than the fuel cost new (then, I
suspect, just polish it or mix it with good fuel and sell it).

--
My opinion and experience. FWIW

Steve
s/v Good Intentions