Back in 1981 I was the base fuels officer at an Air Force base in California...
We got a call from the civil engineers, saying that they discovered an underground fuel tank that dated to WWII. It was in a part of the base that had little to no activity since 1953 or so, and the records were lost so nobody knew when this 50K tank had been filled, or with what.
So, we pulled a sample of the fuel. Oddly enough it had no water in it (that part of California had the water table at about 15' deep, so a dry fuel tank was an oddity). Sent the sample of what we presumptively called 30+ year old #2 fuel oil to the lab to be analyzed.
The report came back and it was within tolerances for brand new DF-2. We pumped the fuel into a large storage tank and mixed it in with whatever was in there (300k gallons or so) and called it good.
As long as there is no water, so no algae or biofilm can grow, it's hard to hurt diesel or heating oil.