I often keep mine in 3 or shift down manually when coming up to a steep-ish hill, having learn to drive a manual (or stick) car, considering the narrow power-bands and that they don't mind high revs, I'd rather be in the right gear to start with than waiting for the car to do it when the car is starting to bog-down.

Shifting should be at around 2-2.5k anyway.

It's recommended in MB literature and I drove my past 300D like that for the 85,000 or so miles I had it for and the transmission still shifted perfectly when I sold it.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys