Thank you! I got it now.
I was changing in Beauty to Z-depth the OctaneRender Viewport and then I changed the Z-depth max. Any idea why the Z-depth viewport isn't working?
So I guess the workflow is:
Use Kernel type Info Chanel
Check nonlinear tone mapping
Change Channel to Z-depth
In the viewport set as Beauty you see the changes you do to Z-depth max
When you're happy use the same value on the render element
Save out using render output in the common tab?