I'd bet replacing the valve stem seals will mostly if not completely stop the oil burning. If you use the right valve spring compressor it can be done pretty easily without removing the heads. Just remove the intake and the valve covers carefully remove the cam caps and use a compression gauge hose hooked to the air compressor to hold the valves up. You can usually tell if it's the valve stem seals just by looking at the plugs.

As stated above the PCV system can suck in oil as well. In my experience it's not really an issue on the newer VCs. You can tell the difference by the location of the PCV valve. It's screwed into the VC on the older 3.0ls instead of the intake. A 91' should have the revised ones.

I'll admit I'm guilty of using RTV instead of a gasket on the upper intake plenum. Sure makes it a b***h to remove.

It's probably a good thing that it has no EGR with it burning oil. There was a layer of gunk at least 1/4" thick in my vans original plenum when I opened it up to port it(original engine burned oil for at least 50k miles).

Cool car!