Ok I see.
So what I got from your explanation that System Families in Revit CAN NOT be modified this is an issue with Autodesk itself.
However I wonder if there's a way for octane to read pipe material not from "Materials and Finishes tab" but from "Material" tab embedded in "Mechanical properties ...

Revit file is in the attachment. (zipped)
Thanks in advance.

Hi,
I am using Octane for Revit 3.7.0.51 and Revit 2018.2 update.
I am experiencing weird material selection issue.
When I select a pipe in Revit, Octane does not detect whether the pipe selected and so shows no material for it, as the following pic shows :

http://i64.tinypic.com/t5ncso.png ...