I hear that now-a-days storing fuel is not a good idea because of ethanol and it only lasts a few months.
1) If one stores fuel past it shelf-life, would adding fuel cleaner and/or fuel stabilizer help preserve it a little more?
2) If one uses bad fuel but if one puts a little bit of it in a vehicle from time to time with a fresh fill-up and then drives, would that hurt the fuel pump, fuel systems, or the car?
You need to add fuel stabilizer while the gas is still fresh. I do that for gasoline stored over the winter, both in gas cans for use in small engines and in the gas tanks of cars that get little use. Never had a problem.
If you want to use bad gas a little at a time I'd do it in a lawn mower rather than a car.
Put fuel stabilizer in the gas before you store it. Newer cars aren't as tolerant of bad gas as old cars. When I bought my '79 Catalina last April, it started on the 5-year-old gas that was still in the carburetor with a little coaxing from starter fluid. I've put a couple thousand miles on it or so since. Modern fuel injected cars don't like old gas even more than a carburetor. It throws off the computer, easily clogs fuel injectors and just makes a mess of things. You can get an old car with a carburetor going again in a day or two if you have bad gas, most everything, like fuel pumps, filters and stuff like that are easy to get to. Not so much on modern cars. Diluting a very small amount of old gas in a full tank of fresh fuel probably won't hurt a modern car. There's less risk to put it in something with a carburetor, like a lawn mower, though. They're easy to clean out.
