<?php 
    if (get_post_type( $post->ID ) == 'post-type' )
    update_post_meta( $post->ID, '_last_viewed', current_time('mysql') );
?>

<?php
$args = array(
    'post_type' => 'post-type',
    'posts_per_page' => 5,
    'meta_key' => '_last_viewed',
    'orderby' => 'meta_value',
    'order' => 'DESC'
);
query_posts( $args ); ?>
<?php if( have_posts() ) : ?>
    <?php while( have_posts() ) : the_post(); ?>
        <h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
    <?php endwhile; ?>
<?php endif; ?>
<?php wp_reset_query(); ?>