My old '79 300D's oil gauge did that for years, new oil or old. Eventually the gauge started to leak and I replaced it. The new gauge didn't do it. Then the new gauge started to leak and I replaced that. The next gauge didn't do it either so I don't think that the leaking was the cause. I don't think it's related to the oil necessarily either. You could just have an old gauge that is flaky at the lower pressures. My old car is still on the road and going strong (my friend owns it) so I wouldn't worry too much.

Frank

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'82 300SD