Hi Matadem, there is not currently a way in MOI to set different line thicknesses, other than for visible/hidden/silhouette lines when generating a PDF, AI, or SVG hidden line drawing.

That's generally something you'll need to set in a 2D drawing program after exporting out from MOI.

You can set the default thickness used for the viewport display in moi.ini though:
[View]
LineWidth=1.0

- Michael