Hi Ironice - you can use the Flip command in MoI to flip an object, some more info here:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#flip

> And this, if i invert the object in SI, it looks like this. Other
> objects with the right normals looks much better and there
> is no dark triangles.

That looks like Softimage is discarding the imported vertex normals when you do the flip - you may want to report that as a bug to Autodesk because instead of doing that it should just simply flip the vertex normals as well rather than discarding them and recalculating new ones from the polygon faces.

Mechanical type models don't work very well with automatic vertex normals where the normals are just computed as an average of all surrounding faces, it usually works a lot better to have the vertex normals from the original CAD model to be used.

- Michael