Hi all, retired tech here. Yes, it will be the silicon contacts worn away. I have repaired many of these contacts over my working life, not only in keyboards, but remote control hand pieces also. You can use alcohol based cleaners yes, never WD40, but the proper fix is to buy a little syringe of silver conductive paint, very cheap off Ebay, around $2.00 AUD that is used for this purpose.
. Wont last forever, but will get a bit more use from it. While you are at it, clean all the carbon pads on the board strip also. Best to use a cleaner that is used for pots (volume controls). Just spray a little on a cotton bud and go over the pads. Just dab a little bit of the paint on each of the worn silicon pad tips and the jobs done. This is a much cheaper fix than replacing the whole silicon strip with a new one, and for an older keyboard, it is far more economical. Good luck with it.