I don't think that idling for a long time would have any negative effect on a automatic transmission but maybe things are different on a M-B diesel trans. If you want to warm up a transmission, usually you have to go through the gears forward and reverse with your foot on the brake.
In regards to the way the transmission shifts on the previous replies, my '85 300D shifts like any other auto transmission when it's working right. I've had some issues lately with it almost skipping third gear under light acceleration. Anyway, when it is working right, I don't have to let up on the accelerator pedal to help it shift but it does sometimes smooth out the 1st to 2nd lurch.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012