Welcome to the fire, Horus!
If I had an unused heating oil tank in the basement I'd seriously consider using it to store fuel for a diesel generator. From what I've read (no first-hand experience) diesel generators have a higher initial cost but give you the best energy-to-fuel output. My understanding is they also have lower maintanence requirements. Danger-wise, I'd put diesel equal to heating oil as far as storage safety goes. If you've been storing heating oil in your basement for years, diesel isn't much worse.
That being said, try to track down the tank's manufacturer and discuss it with them. Diesel might attack gaskets or washers differently than heating oil or something like that. I'm not too concerned about that, but it doesn't hurt to check.
How big is the tank? If it's something huge like 350gal I doubt you could afford to fill it full of diesel. Leaving it half empty would expose the diesel to air, which isn't great for long-term storage. Maybe the people who swapped out you furnace have/know where you could get a smaller tank. Just a thought.
-Blast, who isn't an expert but occasionally plays one in magazines
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