10 years with this tube, and the day has finally come. RIP.
I have had this tube of MX4 since 2013, it's served me well, countless rebuilds of my computers, CPUs and GPUs alike, home servers, gaming computers, laptops, games consoles, pi4, I used this on everything. You served me well.
I didn't notice any breakdown of the paste. It typically dries out over time once applied, this was still the same consistency throughout its life (in the tube)
This has been my experience with MX-4 as well. It's really really really stable. I've never had to re-apply MX-4 due to it drying out, either. (It's supposed to last at least 8 years in use.)
The same tube has been sitting in my drawer and every time I've needed it, every few years, it's been good.
I have decade old tubes. Maybe some bits around the entrance can dry out if you didn't clean them after use but it doesn't seem to be an issue for mine as far as the inside stuff goes (of course I put the cap back on as well).
No, but you do want to occasionally rotate them around. They will separate over long periods of time.
If you have some expensive stuff that has been untouched for like a year, plunge it all out on a cleaned piece of glass, very, very, thoroughly stir it back up, and scoop/smash it back into the back side of the tube and then let it sit upright somewhere for about a month so all the air works itself back out to one end before using it again.
It's not something you'd visually notice all that well. Just that the carbon particles (using mx-4 as an example here) will drift downwards instead of being equally dispersed in the paste. That can leave your solution you apply having too little carbon to do its job as well as it should.
No pastes are just liquid. They're all super finely ground solids suspended in a liquid, and the solids never weigh the exact same amount as a liquid, so given time, the always start to separate.