Annually or every 20,000 km.
There is a filler/level plug on the differential housing that must be removed to check the fluid level. If the vehicle is raised to allow access to this plug, make sure it is securely supported - do not crawl under a vehicle that is supported only by a jack!
To check the oil level correctly, the vehicle must be supported level.
1. Remove the plug from the differential. Use a hex key to unscrew it.
2. Using your little finger as a dipstick, check the fluid level - it should reach the bottom of the plug hole. If it does not, use a syringe to add the recommended oil until it starts to overflow the edge of the filler neck.
3. Install the cover and tighten it securely.
4. If you notice that you need to top up too often, this may only be due to leaks that should be identified and fixed before serious problems arise.