exclude a custom post type with specific term from post navigation

How can I exclude a category term from next / previous post links?

$previous = ( is_attachment() ) ? get_post( $post->post_parent ) : get_adjacent_post( false, '', true ); $next = get_adjacent_post( false, '', false );

I tried to put in the category tag_id like:

get_adjacent_post( false, '8', true );

but this doesn't work at all. What am I doing wrong? A hint would be great!

I also checked the wordpress codex, but I don't get it to work. thank you.