Right, lets clear some things up.
Ron is resistance to pre-ignition (commonly referred to as knocking or pinking), it has nothing to do with energy content or 'quality'. Shell 95 and 98 (for example) both meet the same British Standard quality.
If your car is mapped for 95 Ron, you will get no performance benefit by putting 98 in it. None.
If your car is mapped for 98 Ron and you fill it with 95, the engine may produce less power than it could, if it starts to pink (which is likely). The ECU (via knock sensor) will retard the ignition until the pinking stops. This obviously costs you performance. Once your car has become accustomed to 95, it will take a few tanks to re-adjust to 98 again.
Pinking can damage your engine, and is more likely to occur in higher ambient temperatures. I would run any turbo car mapped for 98 exclusively on 98, especially in the warmer summer months.
So the orignal question was is the M135i mapped for 98 Ron?
i thought a modern engine detected and mapped for the RON currently in i.e.. it would stop pinking before it happened at the cost of performance.