Do NOT use either of those. You will ruin the sliders.
If you must use anything, use one of the few products designed for sliders or faders.
Caig laboratories 'Fader Lube" would be my pick.
And by 'strings' he means that inside the fader assembly will be one or two extra slide strips for the led which is mounted in the shaft.
These are separate to the normal carbon tracks that form the actual fader part.
The fact that they are all uniformly dim means you have an issue with the power supply feeding the fader leds.