I just replaced that on my 1984 300D last weekend while installing a rebuilt turbo. I used a Viton O-ring from my Harbor Freight kit. Don't recall the size, but you will need that kit again. It was very tight, indeed I had to pull down hard and keep wiggling to snug the drain tube on so I'm sure it won't leak and Viton lasts a long time. Yes, I poured oil down the tube to lubricate.

For the turbo drain hole, there is a rubber grommet and an O-ring which fits on the bottom end of the tube, as I recall. Mine were fairly new so I didn't replace, but did replace the O-ring at the half-way point w/ an HBNR one (green) since I had no large Viton ones.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans