I've never heard of fuel losing octane. Fuel gets harder to burn as it ages, up to the point at which it doesn't burn (I poured it on the floor and couldn't light it with a match!). Very roughly put, the higher the octane, the harder it is to burn. Thats why you got the black transom, not all the fuel was getting burned, and you lost a couple horsepower by spending more on gas! A 502 mpi loses 2 hp going from 87 to 93 octane! But I digress, if anything the octane would be rising, although its really the ried vapor pressure thats changing.
Now forget all that stuff, put fuel stabilizer in your tank and top it off if its going to sit longer than a couple of weeks. Then your good for up to a year or more!