The fluid is most likely transmission fluid that travelled up the speedometer cable. Don't ask me how that happens, it just does. Pull the instrument cluster and you will probably see cable is wet with the same fluid. And as another poster said, oil pressure sending unit is electrical, not the mechanical kind with an actual tube with oil going to the gauge (which could spew hot oil on you in a wreck, which is why they don't use them).

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car