I always leave my 300TD in 'S' for the first mile or two after leaving home in the morning. Sometimes while driving around town I'll leave it in 'S' as well. You can reach for that turbo a little easier when you need it. On the highway always in D. The transmission when warm shifts smoothly when using light to moderate throttle and sharply with an insistent request. The diesel is a little more demanding on the tranny though. Car's got way over 300K on the odometer.

I live in Eastern Kentucky and we have plenty of curvy, sloped roads to test the shift points in any automatic transmission. I would shift it down if I knew that a stretch of road requiring a little more control was ahead, maybe more so on wet pavement.

My 89 300TE does not seem to require the kind of finesse with the shifter as the TD does at times. The TE seems to provide sufficient engine braking to assist a light touch on the brake and willing to downshift with a quick (yet brief) stab to the throttle.

Bottom line IMHO shift it if you NEED to shift it. I don't think it hurts the drivetrain at all. If it were a FORD, (and I've had a few) I would definitely think twice before demanding the limits of the engineering.

Use it, don't abuse it.