Correct Trans Fluid for '86 4KQ?

[Mustang]

Hello,

My name is Brandon Traynham and I purchased an '86 4000CS Quattro about a
month and a half ago as a winter driver.  It has a great body, paint, and
interior, but high mileage.  All the major components of the driveline are
in good shape, but it needs most of the maintenance items replaced.  So far
I have replaced the shocks, wheel bearings, bushings, brake rotors, pads,
etc.  Now I'm turning my attention to fluids, filters, and other tune-up
items.

I've been just reading and learning so far on the list and I appreciate all
the good stuff I've learned so far.  I do have a question now and after
searching the archives, I was unable to find a definitive answer.  I'd like
to know what is the correct fluid for the transmission/front differential?
A couple vendors recommended the Lubro Moly Voll-Synthese Synthetic Gear Oil
(75W090), but noticed it is GL5 rated and the Bentley manual says the car
needs GL4.  The posts in the archive seem to be evenly split between the
Redline MT90 and MTL with a few folks recommending the Audi synthetic lube
from the dealer.  I did notice that may of the archived messages dated back
to '93 so I thought I'd ask to see what the current conventional wisdom is.
Thanks in advance!

Brandon