'90 200Q Transaxel lubricant

["Greg O'Malley" <gomalley@us.ibm.com>]

Hi,
    I have a real slow leak in my '90 200Q transaxel, and I need to top
it off...
Bentley says Synthetic GL4 75W-90.  I can find GL5 synthetic, and GL4
blends, but I can't find the right stuff locally or by mail order unless
I get the AUDI oil from the dealer. (@$20/liter, pretty steep for gear
oil, but....).
     I'm inclined to top off with the GL5 synthetic.I'm a little
concerned about the synchro's and the torsen.  Does the substitution of
GL5 cause a problem?  Does someone know of a Synthetic GL4 75W-90
outside of the Audi Oil?
Thanks.
Gordon Malley.