It sounds to me like the only way that the fuel could have caused the rich running condition is if it caused some damage. I am suspicious of this being the case.
Incorrect octane has nothing to do with fuel mixture. If the octane is inadequate, you will get preignition and possibly the damage that goes along with preignition.
If the fuel caused the problem, it's too late to correct it by changing fuel.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
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