Our 2.6L M103 has been in the family (I had it, Dad had it, I have it back) for 260K of it's 340K-kilometers (about 220K-miles). The oil used is Quaker State dino juice, changed every 7-8K.
The engine uses little oil, does not smoke, and has lots of power. It shakes at idle, but not due to compression loss, but a set of engine mounts I have not had time to change.
I figure the engine has reached about half of it's practical life, maybe less.
1998 C230 "Black Betty"