Stevebfl frequently posts that on M103 engines, valve seals should be replaced about every 90,000 miles. Valve seals only, not valve guides. This is good news as valve seals are much cheaper to do because the head doesn't need to be removed. Replacing valve guides requires the head to be removed.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".