My money is on the cam cover gaskets too, given this is a very common condition on the V6's.
I suggest removing the plenum (excellent guide on here), cleaning the breathers (I would do that on any new acquisition anyway) and while you are there pull the plug leads and check for oil on both banks - also have a good feel around the outside - they can leak either outwards (results in burnt oil smell as oil drips onto exhaust and that all Omega owners are familiar with

) or inwards into the plug wells, or both. If you are really convinced they are ok then look elsewhere. It is an averagely painful job to change them esp passenger side, and not for free (c. 50 quid for the gaskets, sealant etc), but if leaking they need doing or will require even more painful and expensive jobs elsewhere in future.
While the plenum is off , worth changing plugs if that is anywhere near due, and have a look at the HBV round the back to check that is not showing signs of leaking coolant. If its not, it will do sometime and its worth knowing where it is.
BTW remember that gravity will result in the leaked oil moving generally downwards, so if you see it on the sump rim it just means its coming from somewhere above (and not necessarily straight above either..)
Good luck!