It seems to me you are acknowledging the possibility of flying your aircraft with less than the Rotax required 95 RON (thereby validating my practise) - your 95 RON (if it even was 95 from the pump) will degrade, which means it can only go below 95. On the other hand my 98 RON will also degrade but may only get as low as 95 in a few weeks - a few weeks more it will be at about the level your 95 has now dropped too. Possibility is that we are both flying with the same RON - yours just cost less to degrade than mine. It also troubles me that you might be attempting to use fuel that has been open to atmosphere and is 6 month old - he/she who does that in an aircraft, may not wish to live very long. I am not a chemist, so my tentative opinions are based on what I have heard and read - all fuels "go off" mainly from the volatile fractions being lost as vapour - "evaporating" if you will. This can not happen in a closed container (other very much slower deterioration will occur). All fuels will degrade over time. This will occur more rapidly if the fuel is exposed to the atmosphere (open container) and is influenced by area expose, agitation (which is area) and temperature. I have a sneaking suspicion that sunlight may also be factor - hence opaque containers stored in dark places seems to be the norm. The small "breather" vent on most fuel tanks will allow the volatile fractions to escape but not as quickly as say the tank cap being left off (surface area exposed to atmosphere). In theory fuel tanks should be kept full (minimising surface area evaporation and water condensation development) or near empty, so that fresh fuel can be added befor each flight - practicality is another thing.