What is the minimum AKI requirement for the s63? This should be in your owners manual or on a sticker near or around your fuel filler flap.
If it is 89 AKI...then when you get to half a tank...put the highest AKI in your area (91 or 98 AKI...this will bring the fuel left in the tank up to the minimum (if the minimum is 89 AKI).
Your owners manual also should tell you what happens when you use fuel below the “recommended” fuel spec (lower performance & higher gas consumption).
Your engine also has knock sensors that help when low grade fuel or incorrect octane is used. Here’s some info from the TIS about the knock sensors...you can also go to the site, enter the last 7 digits of your VIN...then type
knock control into the search box for even more info:
https://www.newtis.info/tisv2/a/en/f...ensor/6LPkwe8E