I acquired an aftermarket diff cover with the extra oil capacity thinking it would be a good safety measure. These things are not a very well thought out design and suffer from one huge issue. The baffles on the inside are all in the wrong place. The vertical plates are too far in from the edge of the diff center and the upper most horizontal plate is too close to the breather pipe. This results in a lot of oil being flicked up into the top most recess where the breather is located. It only takes a few mm of oil to get flicked up into that tiny recess and it starts blowing oil out the breather. It would loose about half a litre of oil over a week end event and make a huge mess under the car.
I tried all sorts of ideas to put the breather up higher and restrict the flow out of the breather but none of that worked.
In the end I had to add a splash cover inside the diff cover and fill the rear baffles with stainless steel wire wool. I also drilled a hole in the bottom of the uppermost breather cavity so oil could drain back down inside.
As an extra precaution I fed the breather hose up into a catch can mounted inside the boot. It turned out that this was not required as the splash cover inside the diff cover worked perfectly and no oil ends up in the catch can. I left the catch can there as its very useful for filling the diff, its slow but easier than doing it from under neath the car.