I recently drove my 1982 300 TDT over the Kern Plateau to the San
Joaquin Valley.  You climb 6000 ft to the top of the plateau on a
winding, steep, narrow road.  Then drive 50 miles at elevation across
the plateau to the rim of the canyon of the Kern River at 9200 ft.
Then drop down and incredibly long series of hairpin turns and steep
slopes, requiring low range almost the entire way to 3,500 ft.  Then
the road climbs up the far side of the canyon another 5,000 feet or so
and then drops nearly 7,000 feet, again on a very winding, low gear
road to the San Joaquin valley, then the road winds through the scrub
oak ranches for 30 miles and then climbs back up another 7,000 ft with
another hundred switchbacks or so to the elevation of the Giant
Sequoias of Sequoia National Park.  I had the AC on when necessary and
the car was comfortable the entire way despite the humid, 90+ degree
temperatures at the lower elevations.  I stayed overnight, did some
photography work, and then returned back via the same route the next
day.  When I got home I checked the oil and the 3 liter, 5 cylinder
diesel had consumed about a cup of oil in the 500 miles of grueling
driving.  The transmission was really put through its paces and didn't
use any oil.  It also does not leak.  The only oil leak on the engine
is a little bit from the valve cover because I didn't replace the
gasket the last time I adjusted the valves.

I bought the car when it was ten years old and had 144,000 miles on
it.  The original owner had just put in the third transmission in the
car and decided to get rid of it because it went through
transmissions.  I have faithfully changed the oil and filter in the
tranny every 15,000 miles when I was running dinosaur fluid and now
that I am running Mobile 1 synthetic I change it every 30,000 and this
third transmission still works like new.

So I would say that the 1982 300 series cars were very well built.
And ride and drive like a dream.  I have driven the same route with my
4Runner and I am exhausted after just one way, let along driving back
the next day.  When I drove the Benz I was not even tired when I got
home, and the car got over 30 mpg despite the incredible climbs and
descents.