The Michelin ProTek Max tube I installed two years ago developed a slow leak this year, which I eventually ascribed to the valve stem, because the sealant should plug any other leak.
Cutting it open reveals the perfectly good greenish-yellow sealant:
The sealant also carries black rubbery grit / shavings / dust, perhaps intended to jam inside larger gashes while the sealant coagulates and binds it together.
There’s a lot of rubber floating around in there:
Dismantling the Presta valve stem show the rubbery crud on and around the valve seal and seat:
Whenever I pumped up the tires, I finger-tightened the nut to ensure a good seal, as you do with all Presta valves. Obviously, finger-tight can’t handle that much crud between the sealing surfaces.
I’m sorry to say I was right about the leaky valve stem, because I think all the ProTek tubes will fail in exactly the same way.
The valve has small wrench flats making it easy to remove, so I can at least attempt to de-gunk them when they develop slow leaks.
Color me unimpressed.
The Smell of Molten Projects in the Morning 
Those rubber fragments look like they’ve worn/corroded from the inside of the tube rather than been included in the sealant. Was the inside of the tube pitted?
Nope, it was in fine shape all around the outside and in the sections I dissected. The lower picture shows the inside surface of the tube: it’s not shiny-smooth like the outside, but it’s not chewed up, either.
Definitely a puzzlement. The other tubes seem OK so far and I hope I just got a manufacturing defect … but how would they get that much crud inside a tube?