I have torn apart 100s of magnum motors and only see them on 5 speed equiped trucks from the factory, supposed to keep it from jumping if you park pointing up a hill with the trans in reverse or vise versa. If you acidentally let it pop backwards it can make it jump when the chain gets sloppy. The 318 in my dakota with 5 speed came with one so they are not V6 only. Also they never used them with a double roller chain only the one with all the links sandwitched together so lots of surface area to absorb the load.

Based on all the guys with problems VS no problems I would say don't use it but I have never used on on anything but the factory style chain in a rig that came with one from the factory.