Everyone seems to be missing the fact that it looks like the leak is at the
oil sensor (based on the linked pictures) the oil cooler is not the suspect
anymore. (although those are all good tips for doing that o-ring
replacement).

I have never replaced that sensor before but I hear it can be a bear because
of the tight quarters in there. I think (although I could be wrong) you
might find something about this on ben's website:  http://www.benplace.com/

--
Thanks,

Jeff
'85 GL (Gertie)
'90 Carat (Grover - the noble parts donor)
'86 (We call this one Scrap)
'78 Bus (Melissa) Patty's Bus