The oil in a modern diff is usually for the life of the car and unless there are problems or oil loss is not normally changed. Perhaps for cheap insurance it could be changed at 100,000 miles or 150,000 km. Up to you but the intervals are normally very long unless the diff is subject to very heavy use or contamination such as may happen in 4WD vehicles that often ford rivers and/or operate in severe conditions.
Peter,
I think you are confusing bad advice from some manufacturers and garages winging it on diff oil changes for facts.
Any investigation of used oil analysis on diffs shows that at around 30,000 miles there is in the region of 300 to 400 parts per million of wear metal in the oil and if you check the viscosity, the oils are in most cases well below their lower limit.
With you having a liking of Jaguars, certain versions of the around 2003 XKR had a specific Shell diff oil only available for Jaguar dealers that was "filled for life". There was no accessible drain plug. The oil had specific additives to prolong it's life. No I did not find any UOA of these diffs.
A few years later Jaguar went back to having fill and drain plugs on their diffs. i think the reintroduction of these facilities on the Jaguars speaks volumes.
Volkswagen and Ford are at it again on their smaller cars with "filled for life" gearboxes and reports of the oils proprieties at say 50k miles are not good. In saying that the car is probably with it's second or third owner and therefor the manufacturer is now off the hook due to missed services on that one.
