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.