I have a 1983 300cd with 203K miles. My air filter was changed 2000miles ago and has about 25% covered with oil. When my car is at idle I can lay a paper towel over the oil filler hole and it won`t blow off but when I pull the throttle linkage to rev it about 4k I have to hold the paper towel down with my hand. I can feel the the puffs of air through the paper towel and the towel get damp with oil.
Not having a compression check yet does this sound normal or excessively worn for 203K miles?
There are a handfull of issues with the car I understand are typical problems with the W123 ie airconditioning not working, dash lights not working, some vacuum issues, etc. I want to fix the car up as I understand the 300cd`s are less common then the sedans. If the engine is dogged I might not put much into the car. If I think I can get at least another 100K miles out of the engine I might partially restore the car.

A few months ago there was an experimental thread on blowby which was collecting data on blowby by seeing how long the engine took to fill up a 30 gallon trash bag. You could conduct the experiment on your engine and compare it to those results. Your blowby sounds minor to me.

There is also a specific tool available to measure blowby. I've never seen one.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

It would be interesting to see some quantified data on blow by as everything I have experienced is anecdotal. Typically though, at idle blow by is not objectionable until, once you take the oil fill cap off the valve cover with the engine running, you have to make a real effort to get it back on, not because of the shaking either. Typically there is visible air flow and noise. But putting the cap back on is not like trying to screw a hose spray nozzle on with the water running in the hose. When that is the case, there is typically more blowby than can sustain the engine compression, especially cold, and you get some additional symptoms, like poor cold starting.

If you have no other issues with the car the blow by is likely not a real issue. Jim

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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)