I bought '79 300SD for a parts car and it ran so good I've been driving it for 3 years now and 33,0000 miles. It had 160,000 miles on it when purchased. Since I wanted to preserve the engine in case I needed it for my '80 300SD, I put in Castrol Syntec 5W50 because I use it all year round.
Originally it used 3 quarts in 10,000 miles (change interval), then 2 quarts and now at 93,000 miles (3,000 miles after last change)it is barely off the full mark. So the synthetic seems to be improving the engine. NOTE: I also add about 4 ounces of PTFE at each change whch may also be partly responsible for the improvement. Here's my motto: "Change it hot at 10,000 miles". I agree that this may not be applicable to cars used for a lot of short trips.
P E H