I've seen this view expressed many times on the forum. For the past 25 years I've had a policy of buying vehicles like this. Nice cars bought by upper middle class people upon retirement and driven very little. Usually they are at least 15-20 yrs old and typically less than 30k miles. I've never had to replace a seal on one of them
My 77 300d which I bought in 2001 from the original owner had 55k on it. No seal problems whatsoever.
I think it's a myth.

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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