I put 5 gal of 1 (or 2, cannot remember) year old gasoline to an old Chevelot (350 engine) and filled it up with fresh gasoline + 1/2 qt. of Gumout. It was leaded gasoline back then. My next two week was a nightmare. The car could not start the next day after about 100 miles. I used starting fluid to try to start it and broke 3 push rods. I took the heads out to a rebuild shop and changed all valve lifters and rods. Still running with the same tank of gas, it happened again after another 100 miles. This time, no starting fluid, but pulling the intake manifold out to find out the reason - there was a lot of brownish sticky stuff at the valve and the seat. It was so sticky that the valves were very hard to open. I cleaned them with Gumout and put everything back together; replaced remaining gasoline with fresh fuel. Problem was resolved.
Do not use the old gasoline to clean parts. I read one true story about a guy who was doing just that in a supposedly safe place. He got burned because he dropped a wrench on a concrete floor and the spark ignited the gasoline. -- another reason to drive a MB diesel.