The shut down test is very unreliable in my opinion. A leaking gasket will lengthen the shut down time by decreasing pressure on a high blowby engine and tight gaskets on a low blowby engines will cause it to shut off quickly.
Precise measuring with a calibrated blowby measuring tool may have some relevance but until you do that and compare the results to engines in a variety of known condtions of repair/disrepair, I'd find something else worth worrying about.

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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