No one is telling you exactly why you should fill up with 91+ instead of 87. I’ll try.

Your engine runs a high compression ratio and a turbocharger, which requires fuel that doesn’t self ignite under high pressure. Higher the octane level, higher the pressure required for self ignition. So if your car states you need 91 and you use 87, your fuel is igniting before the spark plug even lights up (or midway during the compression stroke). This adds excess heat and stress on components in and around your cylinders.

“ECUs are smart enough. They adjust” - how does an ECU alter the compression ratio? Compression ratio is a function of bore and width of the cylinder and stroke length of piston. ECU alters other parameters trying to ‘minimize’ the risks. It’s supposed to be a failsafe and not for regular operation. In any case, your engine is running in less than ideal conditions when you fill up with 87.

Hope this helps.

Lightly used

I had the exact same question when I had my G20 330i and it drove me nuts. After a lot of observing while driving, I figured the inner bulbs turn on when you’re turning, to better illuminate the area you’re turning into.