Blowby refers to gasses that pass by the piston on the compression and power strokes and enter the crankcase. These gases build pressure in the crankcase and need to escape. The hose from the valve cover to the air filter (pcv=positive crankcase ventilation) allows these gases to escape. Very high pressures will cause oil to escape along with the gases. With the oil fill cap removed and the engine running it is easy to get a sense of how much blowby an engine has.
Gasoline engines have blowby also but since diesels run on higher compression, there tends to be more blowby in a diesel engine.

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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