Preload only a post type

WPSC preloads only public post types. It’s better to remove public flag if you can (because they are not viewed by users). Selection of post types isn’t possible by setting but it’s possible to use filter wpsc_preload_post_types which returns array of post types which are preloading.
Do you need exact PHP code?

If you want to exclude a post type, then you could use this filter and unset function:

add_filter( 'wpsc_preload_post_types', 'my_wpsc_preload_posts' );
function my_wpsc_preload_posts( $post_types ) {
   unset( $post_types['question'] );
   return $post_types;
}

If you want to preload only couple post types, then you could use this snippet:

add_filter( 'wpsc_preload_post_types', 'my_wpsc_preload_posts' );
function my_wpsc_preload_posts( $post_types ) {
   return array( 'page', 'post', 'question' );
}