Reading this thread, I am not convinced you have a ring problem at all. You recovered 100 psi but are still down 150 or more psi. To me that is an indication you have another place where compression is leaking. I would invest in the leak down test.

I will note that running my old 1982 240D on Mobil Delvac 1 5W-40 synthetic did wonders for extending the life of the engine. It turned the car from a really unreliable starter from about 45*F and lower, with no starts at all below 30*F to a car that always started, all winter long in Troy, NY without any problems. So, there is likely something to gain from addressing the cleaning of the soot and other particulate out of the ring grooves.

I also second the recommendation to run it hard once you change the oil. Then change the oil frequently (I used 6k miles as the limit with the Mobil Delvac 1, but did no oil analysis to support the interval - "dino" oils are going to require more frequent oil changes) and keep running it hard. The hills in Troy, NY were part of the therapy for my 1982.

Good luck. One cylinder so far down, and the failure to really recover the compression wet is an indication of something beyond rings. But hard driving and synthetic oil are just generally good for Diesels. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)