<snip>
oil is dripping from somewhere over the right (if your looking at the back
of the van) heater box.
<snip>

All  too often, someone trying to get a stuck valve cover off will pry it
off with a screwdriver or some such tool and warp the cover's sealing
surface.  I have no way of knowing if this is your problem, but it's worth
keeping in mind.

Mike