HP 67 cleaning: which silicone grease for the switches?

[Helix]

Hello,
I have acquired a working HP 67 at a reasonable cost (I have the feeling that this very sentence is the most important one of this post )
However, three of the top keys don't work (B, C, E). Furthermore, when I switch on the calculator, sometimes there are weird numbers in the display, instead of 0.00.
I've opened the case, following various instructions I have found on the web (thank you to all those who have posted these explanations!)
The inside was very dirty. I cleaned the space underneath the metal bridges by inserting and sliding paper strips, and I had the confirmation that the top row was dirty. I hope this will solve the problem for the keys.

Now, I have a question about the ON/OFF and PRGM/RUN switches. There was a strange dark paste above the contacts, and I preferred to remove it.
I have read that silicone grease must be applied on these contacts. I have silicone grease for taps/faucets. Is it suitable for that?

Also, I wonder how it can work. My understanding is that grease is an insulating material. How contact can happen if there is insulating grease?