Annually or every 20,000 km.
The differential housing has a filler/fluid level plug that must be removed to check the oil level. If the car is raised to allow access to this plug, make sure it is secure - do not climb under a car that is only on a jack!
To properly check the oil level, the vehicle must be level.
1. Remove the plug from the differential. Use a hex wrench to unscrew it.
2. Using your little finger as a dipstick, check the liquid level - it should reach the bottom of the cork hole. If this is not the case, use a syringe to top up with the recommended oil until it overflows the rim of the filler neck.
3. Install the cover and tighten it securely.
4. If you notice that you need to top up too often, it may only be due to the presence of leaks that should be identified and repaired before serious problems appear.