I could see an argument for using something like Loctite 609 Retaining compound on u-joint caps rather than lube.
U-joint caps spin because when the joint is rotated the cap has force applied to it's circumference unevenly, creating a situation where the cap wants to oblong the hole in the yoke ear. This causes the cap to "walk up the ramp of the oblonged hole" (for lack of a better description) and rotate in the yoke. Its not really a big deal, but can cause problems when the C-clip is seized onto the cap because the act of the cap rotating will cause the clip to push into the yoke and will spit the clip off, causing the cap to eventually exit the yoke. This is why people use full circle clips and/or tack weld caps.
Loctite 609 (or another retaining compound) would help the yoke "stick" to the u-joint cap, effectively pulling the yoke towards the rear of the cap to counteract the "oblonging."
I have no idea if this is a good or bad idea. It just crossed my mind while reading this thread. I could see it adding a very marginal amount of strength to the system.
