Almost all synthetic winch line is made from Dyeema fiber (by Dyneema) or Spectra 12 strand (by Allied Signal). Amsteel Blue is made from Dyneema with a special coating from Sampson. These lines are readily available from both crane & rigging suppliers and marine suppliers, either by the foot, 600ft spool, etc. It is easily spliced, so you can put whatever you want on the ends yourself.
Amsteel Blue is great, but don't limit yourself to it, as there are other varieties out there. The marine and rigging suppliers also have better information on working load limits, elongation, weight, etc than you will find on most 4x4 websites. I would also be wary of the synthetic line sold on Ebay, as you really don't know what you are getting. Do a Google search for Dyneema rope or Spectra 12 strand and you'll find plenty of sources. Check out Redden Marine Supply, Puget Sound Rope, and Rope Inc.
