Question – M.2 pads, when to re-use, replace, etc?

In a PC I built, I had to remove one of two M.2 SSDs and because they were covered by a single ‘heatsink’, I removed the heatsink which had two M.2 pads on.

I have some spare pads, and the pads on this heatsink are looking a little dusty/hairy as the SSD removal happened some months ago.

As I’m aware that these M.2 pads excrete some kind of (non-conductive) oil, I wanted to know if there’s some collective wisdom when to replace them.