This is mostly effective for diesels, but a couple products can be used in gas engines as well. Can't believe I've been using Lucas UCL for this long in everything I own, although most of their other products are snake oil. B2 is the best lubricant for a diesel.
http://dieselplace.com/forum/showthread.php?t=177728
http://dieselplace.com/forum/showthread.php?t=177728