When I had a trench drainage system dug in the basement of my old house, I had to move the old heating oil tank that had been left in place when the furnace was converted to natural gas. I had owned the house for 12 years and assumed that the tank was empty, but when I tried to move it, it was heavy, and I heard sloshing inside. I opened up the drain valve, and out came this umber-colored kerosene, which judging from the installation date of the gas furnace, was 25 years old. I ended up draining 17 gallons of the stuff to finally empty the tank, and I gradually used it up over the next few months by putting a gallon in my Dodge Ram diesel at every fillup. When I put the first gallon in, it showed a tendency to cause the fuel lift pump to stop. I assumed it was gumming up the rotor, so I dosed the old kerosene with Power Service diesel fuel treatment. No further problems after that. All these shenanigans were back in 2016, and I have put about 180k miles on the truck since then, so I think I got away with it.