Our solution to this problem took quite awhile to get to from our end.
Received different answers from various types of businesses.  Finally ended
up with an "old time" mechanic (who works on anything because he's been
around so long), who found that an insert had already been put onto the tank
and he opened it up and drained and changed the oil.  Took him about 20
minutes or so to keep working on that insert to free it up.  Other places
told us to take it to a welding place and have an insert put in so that we
could drain the oil from this new place.  Also were told that there are ways
to get the oil out through suction-type devices that suck out the old oil
and put new oil in through the oil insertion area (they supposedly do this
on boats??), but we would have to drive about 30 miles or so to get to the
one place in our area that does this type of work.  It was one of those oil
change places, but not one of the chains.  I just kept calling different
places and finally came up with people who were willing to do this type of
work (most of the answers were "I don't know").  When I called the local VW
dealers (2 of them), they were not even interested and said they had never
heard of this!  Typical for the dealers.  We finally found the "old time"
mechanic from a reference from the local Jiffy Lube place, who send their
"problem" vehicles to him for work because he seems to fix anything that
comes along.