I think it would be a good idea to add an 8 string guitar option in the Search requirements dropdown menu. It can be difficult if you have an 8 string and want to only look for 8 string songs. I think even possibly adding a 9 string option also would be a good idea. 

I mean, you can search by tuning, thats how you're going to find 8/7 string songs  since RS cannot natively support above 6 strings.  We have expanded our definitions for tunings and tags for "heavy strings" required, but not all previous songs have been updated for that yet.  thats up to the cdlc creators to update their charts