Mine does the same way too. The gauge needle would rapidly bounce after a good highway run, but mine started doing it last week. I don't know and I'm not sure but I think, it is temperature related. Ambient temp here was 3 digits last week and I don't have these kind of needle bouncing before the ambient temp went to the hundreds, and it doesn't bounce at night when the temp is around 80's -90's. BTW, I just had an oil change two weeks ago and I'm positive that the oil filter shaft o-ring are still good. One way to check is try to hose off the oil cooler radiator while at idle and see if the bouncing stops.

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'85 300D Turbo - CA Version