Don't use synthetic gear oil. It's to slippery and the syncros won't work and you will get lots of gear crunch, been there and did that!
If you want to use a synthetic lubricant use one specifically designed for manual transmissions. Redline, Amzoil and Royal Purple all make synthetic manual tranny fluids.
I have running Penzoil Syncromesh fluid in my A833OD, this is not synthetic though. A huge improvement over gear oil in the ease and smoothness of the shifting and a very noticable improvement over the ATF that cam in the tranny.