se1961,
Excel does all the work in setting up the filter for you.
First, I recommend that you select the "Incl. Default Values" and "Use Obi Version" options when creating the xml backup configuration file (see
http://www.obihai.com/OBiAdminGuide.htm#_Toc333506084).
When you open the xml file in Excel, you will see a header row of column names, one of which is X_UD. You should also see drop-down arrow icons at the right border of each column name. When you click that drop-down, it shows you a summary listing of each unique value in the data rows, and there is a checkbox next to each value. For X_UD, there are just two unique values - "Y" (for this parameter is set at the Default) and "(Blanks)". Uncheck the "(Select All)" option, then check the "(Blanks)" option. The data will then be filtered for just those rows that have a setting different than the Default.
This works as described above for the Windows version of Excel I'm using (Excel 2013). I assume it works the same way for other versions, but haven't tested it on other versions.
Filtering is a very powerful and easy to use feature in Excel. If you find it useful, the Help screens or google searches will show many practical uses for it and tips for how to use it.