as engatwork said - use the filler plug, its exactly like the W124 (driver side of the diff).

The W210 were filled with regular SAE85W90 dino gear oil (the smelliest and thickest kind you will ever come across). If your diff is leaking then dont use a synthetic in there as it will super clean everything out and may make the leak worse.

get some cheap house brand gear oil from any store and top it up. Once you fix your leaks then fill up with synthetic.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)