Gear box oil sealed for life
 

I have the engine and gearbox out ready to go back in, question should I change the gear box oil?
I can't see it could do any harm?
Let sleeping dogs lie or not!

Yes, for sure. Get it changed. The gearbox oil in my 1994 S2 looked horrible!
The diff oil in my Ur looked even worse! I've yet to change the Ur gearbox oil but it's on the jobs list.
You can get new fill/drain plugs on ebay and I recommend you change them while you are at it. You might find they are seized (mine were on the S2). I got around that by welding a bolt in the opening and the heat got it moving.

As Dave says, yes. Change the oil.
With the box out it's easy, when it's back in the car its a nasty job, and much harder to deal with broken filler/drain plugs!

Thanks for that info/advice, the plug in the quattro section is the same size, I've ordered two