having my diff on weekend whats the best oil I can give her guys?

??  The diff doesn't need its own oil - You just have gearbox oil.   Usual 75w90 (I think) is fine and all I run.

that's what I mean't cheers boss

Something like Quantum FullSyn 75/90 is ideal, fit and forget.

how many litres does the box hold?

less than 2 litres from memory.

I found my gearbox a bit notchy/tight when cold with 75/90 in it.

That was put in when the Peloquin LSD was fitted.

Drained it and put 75/80 in which is the standard grade for the 6 speed box and its much better now when cold

I think the VAG standard gearbox oil used is 75W 90.