Mine leaks a little oil in the same spot your talkin about, I noticed my valve cover bolts were pretty loose the other day, and when I tightened those it slowed up some. I think the jest of it is comming from rear main, which is kinda expected on these older trucks.
If your return lines are leaking, REPLACE THEM!! you can get a kit from a couple guys on this site( Agnem, Type4 ) or from an e bay store.(wouldnt suggest that) Your likely going to get air in your lines when doing this procedure, but thats not a big deal. (read the fuel injection service thread on home page) Your oil leak may be mostly diesel if its getting into valley pan!!