If you look at used oil analysis of diff and gearbox oils, the diff oil gets up to 300 ppm of iron in 30,000 miles.
Gearboxes less, say 150 ppm of iron in 30,000 miles.
Yes it is a good idea to change both diff and gearbox oils around 30,000 miles or after 5 years or so as that is the assumed working life oif the oil.
The specification you require is a 75W/90 GL5 oil.
This is thiner at low temperatures 75W and reverts to a 90 grade as the diff temperature increases and the GL5 advises of the Extrem Pressure property of the oil.
Do not use a limited slip diff oil or put in an additive unless you know 100% that you have an LSD diff as the additive tends to a small extent make the oil useful life shorter.
There is no need for a 140 grade LSD oil unless the vehicle is used for racing where the working temperature increases outwith normal.operating temperatures of a normal road vehicle.
