Thankyou everyone for the response - particularly Schlitzaugen. I hadn't through about the details of capacitors since I was playing with electronics and old radios when I was at school, so really appreciated the detailed explanations.Well, he's using this microphone:
You can of course replace the points with a Hall effect trigger and get rid of both the points and condenser. Relatively cheap, easy to install, set and forget. Takes out the wear in the dizzy shaft square sector too, but not the wear in the bushings.
Curiosity got the better of me, so I pulled apart a NOS .27uf ducellier that failed very soon after fitting. This is what I found:
And from the other side:
It is very obvious what has happened here - everything except the copper spacer between the can and the bottom end of the foil core is shiny and covered in oil. The copper spacer on the can side was dry and had a think layer of corrosion on it.
Next step was to test the capacitor it's self - a healthy .31uf
So out with the autosol and off with the corrosion:
Back together with smear of silicon grease on each side of the copper plate - tested again - ta da! back to a healthy 0.31uf inside the can.
So what happened here? It looks like the copper plate was entirely dry on the outside allowing a layer of oxide to form. The rubber top was quite hard, but I doubt this was a factor, as the copper plate is slightly domed, presumably to work as a spring.
Who knows how long this will last - I am nervous about was whether there is any risk of chemical reaction between the silicon grease and the oil or aluminum, simply because I have no idea about the chemistry. I guess only time will tell.
On thing that did strike me was how good this design is from a vibration perspective - no hard connections and everything held together and in place with flexible materials (rubber, paper, copper sheet). Fingers crossed this one will now be good for many years.