Hi m8.
You would be better off getting ally diff cups and cases, as this will reduce any flexing.
Then shim it out to remove any excess play. Check this link http://cgi.ebay.co.uk/ws/eBayISAPI.d...MESE%3AIT&rd=1. They might deliver worldwide if asked.

As for oil, people have their own veiws as to which weight to us, but generally it works like this

Thick oil rear = good acceleration in straight line, but no turn in the corners
Thin oil rear= good turn in, but not as good at putting the power to the ground
Thin oil front = good turn in but will straight line out under power
Thick oil front= less turn in off power but excellent ability to pull you out of a corner under power.

Hope this helps.