Center Differential Fluid for 4000q

[Lines Peter]

There is only one fill plug for the trans...  The same fluid lubricates the
entire unit including the center diff.  There are 2 drains because the shape
of the housing traps some fluid in the rear.
P.S.  Make sure you refill with GL-4 only as specified in the owner's
manual.  The syncros prefer this type of oil.  GL-5 or GL-4/5 combo will
cause the shifting to be difficult.  Only the rear diff. uses GL-5.  Kind of
a pain 'cause you have to buy two different oils!
-Peter

Is there a filler plug for the center differential on the 4000 quattro?  The
drain plug is evident on the bottom.  I have heard different stories that
there is and also that it is filled through the transmission?  I dont want
to
drain it until I know for sure where to fill it.  If there is a filler do
you
have to remove the cat converter to see it?  Also what is the correct fluid?
If its filled through the Tranny you have to use a synthetic correct?  The
Bentley book isnt that clear as to the diff. fluid for quattro versus front
wheel drive.  Any help is greatly appreciated.