Hi Alex:
You dont have to disconnect the oil pressure line as far as I know. I think you can push the instrument cluster from behind by accessing it from under the left kick panel. You will have to remove the kick panel, slide your hand from under and behind the meters, push it out as much as you can, not too much because the speedometer connection is the one that will restrict it from coming out mor. Now disconnect the speedometer cable. It can me done by hand. Once disconnected, you can see the speedometer cable in its housing. I used a long spray tube and sprayed some graphite spray in it end. It made a big diff for now. almost no jump. But if it happens again i will replace the whole speedo cable.

Do a search on the forum many folks have replaced the speedo cable.
hope this helps
MVK

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1985 300D Turbo 179,000miles