Just leave it in drive. There's no real benefit to leaving the shifter in "S" around town. You don't just need higher RPMs and more fuel flow, but you need more air flow as well to clean out the engine, which you get best at a high load, like high speed driving, or Italian Tuneups.

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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.