I found that the out of service fuel oil tank in my old house still had 40 gallons in it 24 years after the oil furnace was replaced by a natural gas unit. The stuff was approximately mahogany colored, so I assumed it had varnish in it. I put 2 gallons of it in my Dodge, and it ran fine, but when I let the truck sit overnight, the fuel pump refused to run the next morning. Apparently, the fuel gummed up the pump, causing it to stick. I tapped on it with a hammer, and got it running, and added Power Service fuel treatment, and the problem hasn't recurred. I have been adding up to 15% of the old fuel oil during fillups for the last couple of months, but always with a dose of Power Service.
Maybe it's crazy, but cheapskate that I am, I just can't walk away from 800 miles worth of (nearly) free fuel.