techinically low octane has more rubbish than hi octane and rubbish donest burn so you have less quality fuel for mileage, so hioctane will give you more miles by a small number.
i ve never used regular fuel in 2e, 4afe, 4age, pk5v engines in Pk if hioctane is also avaliable, only on motorway you have no choice you have to buy regular fuel.
in long run hioctane pays off, by keeping your engines fueling system cleaner. but if you want to use car for short time and sell it off, you dont need good fuel.
in my case i simply didnt use regular fuel only hioctane and drove less , manage trips to avoid avoidable driving on road
