oil leaking from glow plug?

[PowerstrokeAaron]

I noticed i have a very small oil leak that appears to be coming from the back glow plug on the passenger side, i thought it might have been the valve cover but it looks like it is definately coming from where the glow plug goes into the engine? is this common? what do I have to do is there an o ring on the glow plug or something? thanks for any help