silicone "weights"

[Tvak@AOL.COM]

Found some silicone at the local hobby shop.  It's pure silicone, supposedly, 
but it comes in various weights like oil.  30-weight, 40-weight.  The 
thinnest is 10-weight, so I assume that would be best for use in bushings 
(for sluggish butts), but I gotta wonder:  if it's pure how can it vary in 
weight?

Thoughts, ideas, comments?

Tom Sivak