Probably time to have the head overhauled or upgrade to a newer casting. When the valve guides wear out, the valves no longer seat properly and you get compression leakage. If you don't have blow-by problems, the bottom end is still pretty solid meaning the compression loss isn't coming past the rings and into the crankcase.

If you haven't done it already, pull the intake manifold and clean all the crap out of it. My SDL was so choked up with muck that it was breathing through intake runners about the size of my pinky finger.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]