As for FreeCAD 0.18’s STL export I don’t see any options for STL files under Preferences --> Import/Export. All I have options for are: IFC, DAE, DXF, DWG, SVG, OCA, Mesh Formats, IGES and STEP…
You are missing a vital step, then in your export. When exporting to an STL, it is best to first switch to the Mesh Workbench. This will give a lot more control over your exported STL. Going straight from your CAD drawing to an STL leaves too much open to FreeCAD to interpret, and the geometries derived might not be optimal for printing.
Switch to the Mesh work bench. Once there, select the body you want to export (you might only have one, but you still need to select it), then under the “Meshes” drop-down menu, choose Create Mesh From Shape.
Try using standard with a Surfface Deviation at .01, and Netgen set to “Very Fine”. The nice thing about “Standard” is that you can set it to almost any tiny value you want (though I suspect there is a stark diminishing return at anything smaller that .01 – but you are free to test that!). The Nice thing about Netgen is that is can faithfully reproduce some pretty wacky geometry when Standard fails.
Then Hide your cad, and switch to the wireframe draw stile to inspect the meshed product. Once you are satisfied, right click each body and save them to their respective STL files. Using the Mesh workbench can make a world of difference in your STLs… and it is entirely possible that this is influencing your results when going to print.