That shut down test is unreliable and can do damage to your engine seals. I wouldn't draw any conclusions from it. Long shut down times can be accounted for by a leaky seal.
Do you have the early or late vacuum pump? The early pump with the breather line going from the pump to the air filter can cause oil consumption when the vacuum pump diaphragm fails.
Clean the engine off really well at a car wash and keep a close eye for leaks.

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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