It’s an interesting debate.. The truth is the RON rating - 91, 95 or 98 - has to do with the compression ratio in your engine.
Generally high performance engines (read sports cars etc) have higher compression and require higher octane rating such as 95 or 98 RON.
In a standard engine, with a more normal compression ratio, 91 is fine and any higher is simply a waste of $$$.
BUT
I have it from very trusted reputable sources that the fuel we here get down under is mostly very rubbish, especially the 91 and even more the discounted stuff from supermarkets.
A friend of mine who’s a retired racing mechanic swears you’re better off going to BP and use some 95, just because it’s simply better quality fuel. Food for thought?
The truth about E10 (or E15 as it’s now in the states): this fuel has 10% ethanol (alcohol) made from corn in it. It’s way cheaper, hence why they make it.
It’s very, very bad if your car isn’t made for it, as the ethanol will attack and eat into rubber - all your fuel hoses and gaskets for example. Newer cars made for it use different materials such as teflon etc to deal with it.
The other thing about alcohol is it evaporates, and also separates from the oil (petrol is refined oil). So if you leave your vehicle sitting for a while with E10 in it, you’ll end up with very shit stuff in the bottom of your tank, which not only will make your motor run real bad, but also corrode your tank and whatever it gets to. That is why in the USofA, the ONLY place you can buy real fuel without alcohol is at boat filling stations, cos they realised how bad it is for boats that often have fuel sit in there for a long time..
So, I use 91 here in my WBX 2.1 and it runs perfectly fine. Also use 91 in my ‘89 Mazda MX5, and also in my 2018 Subaru Outback 2.6 - all absolutely fine.
But I use BP’s 95 in my bike, Cagiva Elefant fitted with a Ducati 944 ST2 motor. That’s a performance motor, and I trust my mech mate.
Sorry that was long winded wasn’t it?