Unless you have badly worn rings, cylinder walls, or valve guides, you will never see oil in the large crankcase vent hose. All the blowby will be drawn into the engine by the smaller on further back -- only clean air will be drawn from the clean side of the filter through the engine.

Engines with considerable wear will show oil here, this is a very quick and easy way to determine engine condition. If it makes enough blowby to flow backwards, it will need an overhaul soon.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!