I'm on the most recent Oculus integration version using Unity 2018.3.6f1
Is there a place where you can find the default hand materials? I wrote a little script that prints the asset path for the material, but the path was coming out empty.
If you are using the Avatar, I think its pulling them from Facebook. If you are using the 'custom hands', they should be in Oculus-SampleFramework-Core-CustomHands