Center Differential Fluid for 4000q

[CTDiesel at aol.com]

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.