I’m afraid that storing fuel doesn’t work like that, especially if you leave it outside. You’ll get back to a can of varnish by the time it makes any difference
That's good to know (no sarcasm intended) - I was intending on holding onto my modest stash of diesel for a few months at most, but probably worth finding out a bit about how quickly it goes off!
Yeah, fuel degrades over time. There are different numbers out there. Exxon [1] says up to six months. "In general, gasoline should be used within a month of purchase. When the engine will not be used for an extended period of time, it’s best to drain the fuel tank and then run the engine until it stalls. If you choose to store gasoline and follow proper storage guidelines, the gasoline can be expected to remain of good quality for at least six months."
While I'm sure this is good advice, anecdotally I filled up a car with gas just as lockdown started, and then didn't use that car much for the next two years. Still seemed to run fine when I sold it this year.
Diesel goes off fairly slowly.
One year old diesel can be put into any modern car.
Two year old diesel can be mixed 50/50 with fresh diesel and put into a modern car
10+ year old diesel can go into a tractor or an old (pre 2005) diesel car.
Yeah, algae growth in diesel fuel can be even more catastrophic than putting stale gasoline into a fuel system. It can infect the rest of the system. A family member is a farmer, and 3 years ago had a semi develop algae growth in the fuel system. The tank needed to be removed and power washed, the entire fuel system needed to be extensively flushed. This ended up costing thousands.
In general, I wouldn't recommend storing fuel for the average person beyond a gallon for lawnmowers. It isn't worth the safety hazard, and the only way to actually save money is if you are using & purchasing enough that you can contract your fuel purchase (sign a contract to purchase $x by y date) for bulk delivery.