My 302 kmile 1992 300D has original 722.4 transmission.

Which ATF is better for that transmission, new Mobil 1 or Dexron VI (Pennzoil, etc.)?

The newly formulated Mobil 1 is recommended (by Mobil) for Mercedes-Benz 236.6, according to Mobil web page.

Dexron VI (six) has third party approval. In case of Pennzoil Dexron VI, it is not synthetic, and thus would be more old seal friendly.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.