My options are pretty limited over here. However, I've tracked down a couple of litres of a straight 90 mineral oil.
http://www.nulon.com.au/products/gea...fferential_oil
It looks like Penrite has a classics range that I might be able to get ordered in without toooo much hassle (I really wish Australia had some decent auto-parts stores). But, in the meantime if I'm just looking for a top-up solution, I would think this is ok?