I've had the old fuel problem many times,I think it's a lot more noticeable with bigger engines than if you had old fuel in a 3.5 briggs.
I've had a few old cars given to me that were sitting not run for years and drained the old fuel out and as long as the fuel burns and it usually stinks like old paint thinners I would use the fuel in an old car with a single Stromberg carby as long as you go for a drive the same day you put the fuel in and burn it all up you can use it , the only problem was the old fuel has lost octane and lost some volatility, the lower the octane rating fuel burns more quickly while higher-octane fuels burn more slowly so because the old fuel now burns quicker because of reduced octane the motor will have a tendency to knock more and when you turn the motor off they can run on (dieseling).
I have bikes here that I sometimes haven't run them for a while and it's always annoying that I have to pull the carby off and clean it out to get the bike running again.
That's why I prefer LPG ,I have an old Ford here that wasn't started sat in the same spot for 10 years ,it's on LPG ,I wanted it moved the other day so I primed the gas, took the air cleaner lid off sprayed a little 2 stroke fuel down the carby and it fired straight up.
Have another LPG car that wasn't started for a year and that had no problem starting but the LPG in the tank would be about 4 years old.
The only other uses for old fuel I find is for washing parts or sometimes I use it in a spray bottle and spray the ants on brick paths and it's good to put down ants nests and then burn (one way to turn ants into fire ants), works better than an ant surface spray.
I still have some leaded fuel and that was phased out in January 2002 but it still works in mowers and starts easily but when I have some old unleaded it won't even run in a mower and old unleaded completely rusts the tank if left for too long.
