This MB publication says that 1972-74 model year gas MBs should use gasoline with an octane rating of at least 87.
https://www.mbwholesaleparts.com/StarTuned/pdfs/LiquidJune06Star.pdf

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.