Valve guide gaskets

["Jerry C Shaw" <slowtoaccept@yahoo.com>]

My engine rebuilder wants to use some kind of teflon-composite valve guide 
gasket for standard steel guides. He says it will reduce smoking, and, 
hence, oil consumption. He says they are different than the ones used for 
bronze guides. They aren't specified in the parts list or Bentley.

Are there any downsides?

Thanks
Jerry