Peter;
I have a leak at the oil cooler line fittings on my 77 300d so I'd like further clarification on the repair. There is a nipple that comes out of the oil filter housing to which the oil line compression fitting attaches. I assume it is that nipple which has pipe thread and sealer. Two questions: Where would a washer go on that nipple. I can't visualize how a washer would help seal a pipe thread unless it went behind the nut on that nipple that threads up against the filter housing. Even in that position it seems that the sealer on the threads would be the main bulwark against oil leaks.
Why is the job so difficult? It seems fairly simple: remove the oil cooler line, back out the nipple, put on teflon tape or something similar and reassemble. The only thing that concerns me is that some of that sealer may get in the oil galleries.

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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