The zinc oxide stuff is normally oil-based and it depends on what product you use. It is much more like heat sink grease than toothpaste. Once the joints heat up, surface tension sucks the compound into all the nooks and crannies. I usually use the chapstick stuff on close-fitting joints due to its tenacity. The fish oil and diaper rash stuff works well on joints that move against each other due to dissimilar expansion. Neither of these two really harden completely.
OTOH, toothpaste is a hardening product. It doesn't flow out when it heats up. Over time, the water evaporates out, and it ends up like stale polishing compound, cured drywall mud or hazy car wax. Being water-based, it is much easier to rework or clean up later.If the dried assembly is subject to rattling or dissimilar expansion, the toothpaste will eventually lose contact with at least one surface and crumble off where it can.
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