Apologies if the answer is out there somewhere but I cant find it.

I just changed the oil in the gearbox and have a litre of castrol syntrax left over, am I right in thinking I can use this in the diff also. Thought I may as well do both.

Also what size socket will I need to get the drain plugs out, I dont have one big enough and want to make sure I get the right size!

 

As long as you do not have a plated diff (and as not much Subaru's came with one fitted chances are slim you have)you can indeed use the oil.

You do not need a socket, as the plugs have a square hole in them roughly the size of a 1/2 ratchet's so you can use a 1/2 rachtet or breaker bar.

Undo the upper one first, if for some reason the upper one does not come undone you will still have oil in your diff should you need to get to a garage (for a stronger airgun or a bigger breaker bar for example).
If you undo the lower one first, and the upper one doesn't budge you can not drive the car.

 

Cheers got it sorted!

Good tip about undoing top plug first, remember reading that somewhere and did just that.