It seems from the BP documentation, that Octane actually INCREASES while petrol is stored, due to the evaporation of the lightweight volatiles over time.
Quoted from BP:
The remaining components that have not evaporated are high octane and octane can actually increase with time,
but this octane is not available for high revving conditions.
Also, the weight of the fuel increases, and this may affect the fuel / two stroke oil mix.
And remember, we are not flying behind brushcutters or chainsaws that have a very different operational life to a two stroke aircraft engine, which is just running flat out most of the time.
Old fuel does not provide the high speed "protection" that fresh fuel provides.
So, for an aircraft two stroke engine, use fresh fuel, and don't mix the oil until you are ready to use it.
And don't go flying behind your chainsaw.