I need a material which meets the following requirements:

1. If you placed a marble-sized copper sphere on top of a cube of the
material, it wouldn't sink to the bottom, even after 20 years of
sitting in a closed container. This is an absolute requirement.

2. The material must be extremely flexible/elastic, and remain that
way after 20 years of sitting in a closed container. This factor is very important.

3. Hopefully, a very low-cost solution is possible. This factor is
important. I'm hoping for a material selling for $15 a gallon pound or
less.

4. Must be stable at temperature of roughly 85C. Unfortunately, that
may rule out a lot of organic compounds. This factor is important.

5. Must be thermally conductive. Hopefully, will be at least equal to
the conduction of gelatin/jello. Unfortunately, this may rule out a
lot of rubbers. This factor is somewhat important.

The lowest cost solution I know of would be silicone. However, I
suspect that that material would cost $6.50 a pound. I believe the
material is high density though which translates into over $50 a
gallon. If silicone is the only  solution meeting all requirements
except for cost, please let me know the cheapest bet in terms of the
kind of silicone.

If contacting suppliers, ask for a quote for a sample of 20 pounds,
with prospects of higher quantities in the future. Its very difficult
to guess but I would say I will need about 300 pounds a month for the
first few months. I live about an hour from Chicago, IL, in case they
want to know where it will be shipped. However, that information would
not be of that much use because I will need the manufacturer to mix in
an powder additive which I will likely supply them with. Of course
most of the manufacturers will not be available. Its fine if you have
to wait until tomorrow because of these places being closed today.

A polymeric compound is probably your best bet. I need to know more
about the other properties or application before I can give any
specifics. Silicone compounds are good but as you point out are
expensive. Expect to pay $6 and up. However you can probably meet most
of your requirments with some very general rubbers. Your big problem
is thermal conductivity. I need to know what thermal conductivity you
are aiming for. This will define the material selection. The SG will
be linked to this factor. Normal silicones are low density but as you
change thermal conductivity the density increases.
Check out www.bayworth.com as they have specific expertise in this
area and can assist with material selection issues.

By the way. A 20 year life is not unreasonable and used in many industries.

In regards to the thermal conduction, the material may be directly on
an electric motor. If that ends up being the case, it is really
important to dissipate the heat. I would imagine that anything above
the level of water (.6 W·m-1·K-1.) would be adaquate.