Thanks for the info people!

Well, I'll go for the Hypoid oil, as I want the best for my baby

But... Uhhh... Where can I find it for a decent price? The Halfords stuff was a tenner a litre. I'm in West Bromwich, but can mail-order from a site if it's quick enough.
As I mentioned the gearbox is currently in my bath, and I'm gonna fill from the small end, with the clutch/starter end downwards. I'm sure it can't be hurt by
too much oil... Can it?