I can't buy in on your theory about changing fluid in an automatic, anybody's automatic, and have it live a long time. It won't take you long to figure out I'm not an automatic transmission fan.
There is a myth that automatics are faster than manual transmissions. I believe that there are a lot of people that spread this because they like automatics. Probably something like 90% of the passenger cars in the US are automatics. In the 123 diesels, the automatics are not bad, but there is a slight inefficiency in the automatic that uses a small amount of horsepower. With a big V8, you never know you've lost it, and in those applications, it's possible that the automatics are a little faster. I believe that in those applications, the manual would be faster with a skillful driver shifting in most cases.
My wife had a 300TD turbo that
had the transmission fail. I don't remember how many miles, but I was changing fluid and filter every 15K and it still turned loose.
Yes, the little 240D engine screams. It runs right at 500RPM's per 10MPH. This means 3,500 RPM at 70MPH. The original engine in my 240D absolutely defied high rev's from giving problems, at least for a long time. When it let go, I was doing 75.
I'm sure that my half million mile experience was not typical. Most of those miles were at about 65-70MPH. When the speed limit went up, it was really screaming when running 80 on the Oklahoma turnpikes which have a 75MPH limit. At that time I gave it to my son who had just turned sixteen for local driving and bought a '96 E300D. The E300D was a mistake, it had too many gadgets and too much technology which gave trouble.
Anyway, the 240D got 3K oil and filter changes, but they were done with my fanatical oil change method, which I believe effectively cleans the inside of the engine the equivalent of a lesser mileage interval. I pull the plug and filter while the engine is as hot as possible, preferrably on a Friday night so I can let it drain at least over night. This gets more of the gunk and contaminants out of the oil.
As far as timing chain and model years go. There was an updated chain in '77 or '78. If you're changing oil often, I don't think it's anything to worry about.
By the way, my 240D got a patch up overhaul at 380,000 miles. The rear main seal was no longer touching the journal anywhere. At that point it got rings, bearings, a valve job and a timing chain and tensioners. If it were'nt for the rear seal I would not have taken it apart.
Before my son went off to college he let the oil get low. That was only a few weeks before I parked it in August of '98. I took it out of storage and back on the road last November, and it blew in January. I feel that if it hadn't run low on oil, it would still be running. Of course it was slopping a lot of oil and hard for my son to keep up with at the time.
Another thing, mine is a Euro car. I don't know, but suspect that it has a little lower(higher numerically) gear ratio than a US car. But with the manual, and after going through patience apprenticeship, it's not bad on power. I have, however, been accelerating away from a traffic light in Plano, Texas as hard as I could go, and had someone honking at me from behind. I think it's safe to assume that they were asking me to step harder on the accelerator.
The other thing I like about the 240D over virtually anything is it's simplicity. No gadgetry to fail, no electric sunroofs, windows or automatic climate control. No sophisticated oil seperator in the air cleaner to leak like a 300D. It's so easy to work on, I think of it as the modern day answer to a Model A.
I hope all this rambling is of some use to you.
Good luck and have a great day,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in