Can you use an aluminum shim with a carbon seatpost?

Doing a search found it is recommended to use a plastic shim for a carbon seatpost.
Also read you can use an aluminum shim but to put the shim in backwards
so it doesn't grab the carbon post.

Just wondering why it is you can put a carbon post on an aluminum bike
but some say you should not use an aluminum shim.

All I could find on the Easton site was "Do Not" grease the post when it is installed.