RON, road octane number, is not really a good way to specify aviation fuels. (RON and ROZ are the same test) AVGAS is tested and specified by MON rating. RON is generally used as a measure for lower RPM/lighter load uses. MON generally predicts how fuel will react under higher RPM/heavier loads. In the USA they average these (R+M)/2 method to give you AKI, anti-knock index. That said if you are not yet having detonation issues with your fuel it suggests you have not reached the limits of detonation for that application. A higher rating will control the onset of detonation so for that reason a higher grade could be considered a safety consideration. Yes, it will not give you more power however it is better protection from detonation.
If you contact your fuel companies, you will see that they claim fuels will not change enough to worry about obtain values in a 6-month window. There is a big caveat on that when we start to add ethanol, that drops the fuel useable window to 3 months. If you live in a country or region where the blend fuels for temperature, like most of North America, you must be very careful not to use winter blend fuels in the summer months. This primarily is due to the extreme changes within the fuel from the supplier to give you better fuel vaporization in cold temperatures. For most of the USA that is mid-September to mid-April for winter fuels and in the summer months it changes. The primary change is the RVP, Reid vapor pressure, of the fuel. Avags generally is less that 7.8 to 9 PSI which will be consistent all year. Auto fuels however can vary drastically on RVP, from summer of as little as 7.5 up to 9 and winter blends up to as high as 15 (yes above atmospheric pressure)
Ethanol can create some major changes. Once ethanol is blended into fuel it becomes hydroscopic, absorbs water. if there is enough moisture in the air this can start to drop out in a condition called phase separation. Don’t worry too much that is not the worst of it. If the ethanol drops out the real problem is the value of the octane can drop. For a 10% ethanol blend that can be as much as 2 points of the total octane of the fuel. (pure ethanol itself has an octane of 113 and is used to raise the octane of a poorer quality fuel) For this reason never store ethanol fuels longer than say 3 weeks. My reasoning is we have no idea how long it was in transport or storage before we got it for the aircraft, so why take the risk.
Stuck? Not sure what to use? Not sure what is in your tank? The good bit is you can top up with some Avgas 100LL (that is on the MON scale by the way) it will blend just fine and help increase your octane while your auto-fuel will cut the lead within the avgas volume.
So to answer the original question. Use the higher-grade fuels, it is better protection from detonation.
