How demanding are you of your driving? I think it's a waste of time. A well-designed and well-tuned transmission will pick the best gear for the given speed and load demand. Asking for a gear to perform outside of the set transmission design parameters is a waste of fuel. It won't ruin the transmission. Put it in Drive and keep it there. Only under extreme conditions is it necessary to drop it to 3rd or 2nd.

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95 E320 Cabriolet, 169K