Do you remember the Bell innertube that claimed to keep air longer than other tubes? It had no sealant and came with a flattened valve cap?

If that inner tube was able to hold air longer, it was probably very thick and heavy, which would affect the way your bike rides. Those flattened valve caps will fit any inner tube with the same type of valve. There are only 3 types of valves used on bicycle inner tubes, with one of them almost unknown in North America.

Yes, I recall them. They had a slightly lower rubber content than some other tubes which made them cheaper to make but they leached air out more slowly.

So, you ask, why doesn't everyone make tubes like this? Easy. The reduction in rubber content made the tube stiffer and subject to cracking when improperly inflated or as it got older.

Oh, by the way- check for the comments on your other question... about Slime tubes.

No, no I do not.