I've noticed the smooth shifts with synthetic ATF too. The correct AT service intervals is to replace the fluid and filter every 30K miles. In harsh driving conditions, it's recommended to change the fluid only at half that interval.

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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.