If there is one thing to know about lubricants it is this: All Lubricants Fail, and some fail spectacularly.
I have noticed that the use of wd40 in piano actions is widespread. This is guaranteed to ruin the piano and the only long term real solution is to replace the action.
Other than sticky wheels, there is no use whatsoever of wd40 in a piano.
Signs of wd40 use:
If the piano is slow, some notes don’t play and it cannot be played pianissimo, or quietly, suspect this.
It is also more likely on Renner made actions which because of their tight tolerance will require repinning work. In far too many cases, wd40 is used.
This video shows what is needed to make it work again. Thee are 88 notes, three pins on every note, so 264 pins need replacing. Here is just one being done.