The urethane ones withstand freezing better in my experience. The black ones, fyi, are just a regular rubber compound. As far as making a better seal, the black ones have a better overall compression ability, which I suppose would lead to a better seal due to the fact that they sort of "ooze" into whatever you're sealing. The urethane ones seem to do a fine job on tanks, but every time you take a marker apart, it has the black ones it seems. Hmmm...things to ponder...

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