Finally drained the differential.
This is what came out ( Pic below )

Yes, it's dirty like oil.
Doesn't smell good... just like dirty oil smell really.
No burnt smell.
This is what I think I should do.

Add in new, clean gear oil.
Run the car while aloft on jacks ( Not under load )
Drain differential again ( See what comes out )
Replace with 90w.
Run under load for a few under miles.
Drain again, replace again.

Perspectives please

 

You could stir a magnet around the bottom and see if you come out with anything stuck to it which would indicate wearing parts or bearings. If you don't get any particles of metal, fill with fresh oil and just run it.

If you do have metal particles in the oil you will have to take a look inside the diff for bearings and/or wear, but probably both, if the bearings go they throw the diff out and then the wear starts !

 

Does the car have a limited slip diff?

Gear oil does smell really strong even when new, its very different to enginer oil so if it stinks a bit it doesn't necessarily mean that there is something wrong with it or the diff.

If the car does have an LSD then I would strip the diff down and check the clutch packs for wear, I did this on mine by comparing the outer edges of each clutch plate with the wearing surface to see how worn they were and replacing as necessary.

When you rebuild the axle you need both half shafts installed before you can correctly shim the axle.

 

Engaged the advice and took a magnet to a glass jar sample of the gear oil.

The magnet was placed atop the steep cap to magnetize that area, and the jar turned upside down so an particles in solution might settle to the upside down top.

Next day, the cap was removed, and there was some very, very fine particulate matter on the cap.
Normal wear?