$.fn.dataTableExt.aoFeatures.push change setting

I have below code

$.fn.dataTableExt.aoFeatures.push({
    "fnInit": function (oSettings) {
        oSettings.oScroll.sY = 5;
        return  { "oSettings":  oSettings } ;
      },
    "cFeature": "T"
});

$.extend($.fn.dataTable.defaults, {
       //"scrollY": 5,
       "dom":"T"
    });

I can see scrollY changed in function but no effect in datatable, How can overwrite the default setting using this function, since i have to put condition ono tableid,

otherwise I could have done below way which is working

 $.extend($.fn.dataTable.defaults, {
           "scrollY": 5,

        });

I believe I am missing something on return statement which will override the things

fiddle reference

from $.fn.dataTableExt.aoFeatures.push change setting