I strongly suspect the tape is HDPE (High Density PolyEthylene).  Widely
used as low friction surface in woodworking.  But seriously -- yours is the
quietest slider I have ever heard on a Vanagon.  Why are you obsessing over
this?

You have the new-type door.  It is significantly different and better than
the old type.  If you want it quieter than it is, get a set of the Delrin
rollers made by ?Mark Garnick? and sold by I think Van Cafe and maybe GW.

:)
d