Hi Micha, I've attached here a test for you to try to see if a setting for OBJ export for "Write styles as groups" will help.

For this test, in Rhino set the mode for "Import OBJ groups as: Layers" and also set "Map OBJ Y to Rhino Z". Hopefully that should then bring in the mesh onto the original object layers as you want. You should be able to open the original 3DM file, and then do an Import of the OBJ file with those settings and then the import should bring the meshes into the original layers.

The first one is a simple test, the second one obj_test2 tests using spaces in the group names, but as discussed earlier by the actual OBJ file specification any spaces in the group name is supposed to be mean that multiple separate groups are active (spaces are meant to separate multiple names) rather than it being one single group name so that may not work. If that does not work you would need to either ask McNeel to have an additional option to control that group name reading behavior, or have a Rhino script replace all spaces in layer names with some other character like an underscore before doing any of this processing.

- Michael