I have what is now about a quarter gallon of Red Max 5% fuel. I lost it (1/2 gallon at the time) in my basement about 1980 when I quit running 5% nitro fuel. Found it when I moved in 1999. It has been through several months in a hot storage area in Texas, but has not been exposed to light much. It has always been tightly capped. Just for kicks, from time to time, I will run an ounce or so through an engine on the bench. It seems to run OK. In your case of unsealed fuel, I would discard it as you really don't know what you have left.