godslibrary
I believe it will default to FeO. You can change this per selection by setting a selection property named "OxideForms", which will be interpreted as a comma separated list of oxides that you want to use for that selection. You could set that via the selections browser where you can create the property/edit the value for individual selections or an entire group at once. If you're using criteria for the IS, you can configure the oxides there as well.
All the best,