Show related nodes of category selected in current node (using static select list)
I recommend converting your select list field to Taxonomy and use a module like Similar By Terms.


  This Drupal module attempts to provide context for content items by
  displaying a block with links to other similar content. Similarity is
  based on the taxonomy terms assigned to content. Blocks are available
  based on similarity within each of the defined vocabularies for a site
  as well as a block for similarity within all vocabularies.