Here's another thought. Does the turbo oil return tube have a valve in the bottom of it? If so, the oil flow thru that tube might be adequate for stop and go driving but sustained highway driving may be filling up that drain tube, causing a leak at some point.

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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