Just to clear things up regarding all of this,
The octane rating on fuel is its ability to withstand detonation.
In the UK we show The RON value at our pumps (Research Octane number)
In the US they show the AKI value (Anti Knock Index) which is an Average of RON and MON (Motor octane number). this is why you may get confused as to think the octane is lower on american pump fuels.
For example, 95 unleaded in the uk has a typical octane of 95 RON and 87 MON. AKI around 91.
Octane is not especially important for a chainsaw, the most important thing is that you get your fuel from a good source. i would strongly advise not using supermarket fuels unless you really have to as these are known to be the worst.
Regarding Ethanol:
Ethanol is an oxygenator which increases power and lowers emissions, all fuel in the UK at the pump is currently E5 (5% ethanol) but is set to rise to 10% in the near future.
in the US, in most states it is already a requirement that all pump fuels should be E10 and this is what is causing problems for garden equipment in america.
The problem with ethanol is that it attracts moisture if not stored carefully and this is not good for engines. Ethanol is also very corrosive and E10 in america has proven that the rubbers in carburettors in particular do not like this high level of ethanol.
In short, fuel is set to get worse for your chainsaws if your buying from the pumps for your machines.
The solution to all of your fuel worries:
Use Alkylate petrol (such as Aspen) which contains no nasty ingredients which could damage both you and your machines!