Mechanical Layer selection

Dear All,
I've a question about Mechanical Layer selection.
For example, when I try to import 24LC02B/P footprin and symbol, in "Current Layer Mapping" I see the following P-CAD Layer Name and Altium Designer Layer:
1 Top Layer
2 Bottom Layer
6 Top Overlay
20 Mechanical Layer 4

At this point I try to set Layer 20 as "Mechanical Layer 13", but after selection the mechanical layer returns immediately to "mechanical Layer 4".
What am I doing wrong?
What happen if I leave "mechanical layer 4" instead of "mechanical Layer 13"?
Thank in advance
Regards
Stefano