Thrust washer, differential gear

["Larry Hoy" <larryhoy@prodigy.net>]

On the last two rear ends I worked on to get rid of  the famous "clunk", the 
differential gear thrust washer that I removed was
metal (copper, or brass?) and the replacement was fiber.

Tell me, if you can, why the material has changed?  Or is it just my supplier 
that uses this fiber material?  Is the metal type
still available?  Which material is preferred?

Larry Hoy