I used to use Super Unleaded in a previous car because it wasn't designed to use unleaded so the 95 octane standard unleaded wasn't really good enough. The few pence a litre extra for 98 Octane Super Unleaded was worth it to stop the car hopping...
There as been concerns previously about what types of fuels are suitable for particular models of car engines. And the same conclusions appear to state the same- not much difference. I suppose you could go back to the early BP times, when people assumed and believes that BP meant Banish Pinking.
I have a old and reliable Ford Sierra 'general dogsbody' estate, and when the leaded petrol was getting difficult to obtain, I was informed the 'Pinto' engine would be hard to convert, and I should use a mixture of 95 octane with added top-ups of 'the good stuff', with a 'perhaps' additive mix. A friend who owns a garage made a simple adjustment, and its been running on 95 octane ever since, with no noticeable lack of performance or stabilty.
Regarding the 'chip pan' fuel, I have a neighbour who converted a Land Rover and constructed his own 'refinery', but of late he his finding it more difficult in sourcing regular sustainable supplies from the local fry food outlets (Is everyone at it!).
Most modern engines are designed to run at their best on standard fuels as that is all that is available in a lot of countries. This gives the manufacturers a lot less problems than providing different specification engines for different markets.
I often wonder how some of these modern engines survive on out-in-wilds jerry can stocks, especially in some remoter areas!.
Have I ever put a mixture of petrol and diesel in the same petrol engine, and the answer to that is yes. It took the AA three call-outs before they found the problem, and other people in the same area was experiencing the same problem.
Then there's the recent contamination incident, with some of the supermarket supplies!.