Didjaever think that it could be the proportions of something to something in something that determines the properties that are sought as desirable for a particular application? I think the key is butyl content. The five dollar vent tape kit from Camco is not "stretchy" compared to the stuff I used on my rear window. When I mention filaments I mean you can pull this stuff until you've got a "string" 18" long. The Camco stuff will break and leave little strings a 1/2" long.
The stuff I used on my rear window was 1) grey in color, 2) shiny in appearance, 3) stuck to fingers unless moistened, 4) squeezeout could be removed by "scoring" around flange with a plastic dinner knife and pulling off. 5) was not unmanageably sticky and didn't require a solvent to remove or the "self to self" removal described elsewhere, 5) excess was harder to remove after heating by the sun and tended to stretch into impossibly long filaments, 6) "bounced back" when squeezed (like a rubber ball).
I have also used the Camco tape previously. I found that: 1) squeezeout could be removed by method above but it didn't stretch and become more sticky with temperature increase, 2) had an oily feel, 3) a small ball could be elongated by rolling between the palms much like modeling clay, 4) DID NOT evidence the "rubbery" bounceback when squeezed.
I'm thinking from I've seen (not a great deal) and heard here that there are number of products that nominally fall
in this category of butyl tape. Apparently some are a linseed based putty and the "butyl" is only in the label. It also appears that there are some automotive sealants for glazing that come on a roll and have a separator but are a bit too tenacious, difficult to work with and difficult to remove for our purpose (bedding thru-hulls, sealing window flanges).
I wish I could find the shipping label for the roll I bought in 2010 but I can't. To the best of my recollection, a 25' roll was nearly 30$. That could possibly be one indicator of quality and the stuff that's 6 bucks for a 25' roll likely is an oil-based putty which hardens over time. At C World, all bets are off as far as value for payment tendered. It really is sad that the world is full of stuff that's no good and stuff that's too good and some that's just right and it all has the same label!