The age matters too. Anything with carburettors or any older components that contain brass will be destroyed by ethanol. It's highly hydrophilic, so it will rust away the internals of your fuel system.
What? This simply isn't true. There are scads of vintage motorcycles (with brass jets in their carburetors) that have run 87 octane E10 since it's been available at the pumps with no issues whatsoever.
Ethanol does not corrode brass, neither does water. Water can corrode steel, but steel bits are few and far between in most fuel systems for this reason. In any event, the fuel would have to sit for a fairly long time in order for the ethanol to absorb enough water to matter and gas tends to evaporate much quicker than that.
Doesn't ethanol destroy any rubber components in the fuel system?
I have to admit I've been using premium gas in my previous cars (even 98RON in my previous car, but it required 95RON anyway and 98 was a few cents more, plus I liked getting extra range per tank.
With the new car, I've got a tank of regular 91RON unleaded in it. I've been kinda considering putting 95RON in it (because it was built in Europe and that's the standard fuel there, not sure if it was detuned to run on Aussie 91 octane fuel), but it runs fine with 91.
Not sure with E10. The car apparently supports it, so maybe it's worth a try later down the track. It's only a few cents cheaper than regular unleaded though.
using premium fuel is a complete waste.
Not only does it provide no benefits for a car that doesn't require it, higher octane fuel actually has less energy per gallon than regular, so you'll be paying more money to get a slightly lower mileage out of it!
That's the point though, some cars are designed to use premium and will sustain engine damage if you don't use it.
I have a car that doesn't care, as long as I use a lead substitute additive. I did comprehensive MPG calculations over a period of 2 years, and my individual car gets better mileage from premium. The cutoff for me is about 7% dearer (yes, I did drill down that far - stats geek!), after which point regular unleaded is more cost-effective.
It depends on the individual car. The OP's partner's car requires premium ULP, so they'd be risking potentially expensive damage to save a few cents per litre.
Interesting. I thought that the old leaded fuel was roughly equivalent in octane to premium anyway (I believe Super was 96RON), so I assumed that anything pre-1986 would require premium with the additive.