Originally Posted by Mark Johnson
...Also, it never sets up, but stays the consistency of bubble gum. On a hot summer day, & especially on a horizontal surface, it compresses more with heat, and oozes out of the edge more....
There are many different manufacturers of butyl tape, and they can have a wide range of consistencies. Intuitively, I believe that one of the key parameters is the level of crosslinking in the rubber, which anyone with knowledge of adhesive chemistry recognizes as a tradeoff between adhesive strength vs. cohesive strength. (Higher crosslinking increases cohesive strength at the expense of adhesive strength.) The optimum balance of properties can be different for various applications and environments.
I have used two different kinds of butyl tape. The first type (which I bought at an RV store) fits your description: it sticks like crazy but oozes more (especially when hot) and yields at a lower elongational stress. I wasn't totally happy with this for marine bedding, so I bought some of Maine Sail's, which has a little less adhesive tackiness but oozes much less and has a huge elongational yield prior to failure (check his website for a video that demonstrates this). I believe that it has a higher level of crosslinking, and is more suited to the environmental factors and compressive stresses of marine applications.
It is important to choose the right butyl tape, and Maine Sail's seems to strike the right balance.