I'm not sure if this is a S3d or a F360 issue.
I have a cylinder cup shape that I made in F360 that when exported as a STL tries to infill the empty space. If I lay the cylinder on it's side it prints support and does not infill the whole cylinder. I'm not really sure what I'm looking at. I never had this issue when using google sketchup, and I'm suspicious that it's something to do with how it is modeled in Fusion 360, except that if it's laid on it's side the center does not infill.
http://forums.autodesk.com/t5/design-va ... -p/5951017