Cleaning the cable and housing is important. But running the cable dry will cause, of course, failure due to lack of lubrication. Cleaning the oil and lubing with grease will help to block the movement of engine oil up the cable.
The instrument is another issue. The only way to get into the mechanism where you must clean it is to remove the rear cover under the rubber cover. To do this you need to carefully pry up the pressed inside lip of the chrome bezel. You need to go slowly and unlock a little over half of it. Then you can carefully encourage the bezel up and nudge it towards the un-unlocked side and slip it off. It is a bit difficult to do it this way but when you reassemble the bezel, half will be as tight as when is was originally made and it make it much easier to duplicate this tension when re-pressing the lip.
Anyhow, once the glass is removed, take out the two screws holding the mechanisim to the rear cover and take what you can aprt. it is a magnetic drive so besure no steel or iron filings or chips get close to it and simply bleast it with a good solvent. A little grease on the input shaft so it doesn;t run bare- reassemble. It should work like a charm. I was unable to obtain new seals for the engine tach adapter, i am still trying to find a reliable solution.
Let me know if you have any problems.
Allen