On my SD, I found there were a couple of things that helped to alleviate, although not eliminating, this problem.

Make sure the oil drain tube coming from the bottom of the air cleaner has good o-rings. (Ensure the air filter housing mounts are good also.)

Also, the next time you do an oil change, run some "engine oil flush" through that same drain tube. (There is a check valve at the bottom, in the upper oil pan, that gets a lot of dirt and grit in it.) (Do a search for "oil flush and check valve" and you should be able to find a photo, if you would like.)

I believe DMorrison replaced that check valve because, after some testing, he found that was where the oil appeared to be coming from. A little oil won't harm anything, "oil-burners" aka diesels run on that stuff.

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Sam

84 300SD 350K+ miles ( Blue Belle )