You may be getting close to needing valve guides/seals, which I understand many of the older m103 engines need around 100-150k miles. i don't know if MB had changed to the new guide/seal materials by 1991, but it is worth checking out.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E