the diff oil level should be checked when the car hasnt been driven for at least 6 hours otherwise the oil will be areated with small air bubbles and give a false reading
lever off the rubber bung and if anything flows out, let it, if theres water in the axle ( from driving thru water) everything will need to be drained out
otherwise use your finger as a dip stick, put it in and bend it down to check the level
if needed, fill it to the bottom of the hole
since the additives all wear out eventually, its not a bad idea to occasionally change the diff oil. sadly the bottom housing bolts arent drilled thru ( as they are on some earlier holdens) and so wont work as drain plugs, a syringe will be needed to suck the old oil out and to put the new oil in
diff oil no longer seems to have the old familiar hypoid smell, anyone know why?
also check the breather on top of one axle side as if this gets blocked with mud, all sorts of leaks and problems can result