to be honest we are talking about Sainsburys 97 fuel here, Wish people would look at the fact that this is not 97 for all intense and purposes.
Sainsburys use an ethanol mix ( as like most other supermarket suppliers) which is usally 5% ehtanol ratio. But the ethanol as most will know is the main point, as it is just the ethanol that is pushing the octane higher not the other 95 %of ordinary petrol, If this makes sense.
So for example if sainsburys released a 98 octane fuel due to this ethanol use it would not have the same performance results as Optimax
I think this was shown up very recently in the tests on tesco's 99 octane which also has the same 5% ethanol blend.
So the difference between supermarket 97 and optimax 98 is more than you think.
so don't be a tight wad and use the optimax or chicken out and buy a diesel if its too pricey.