How often should the diff oil on a 1999 C280 be changed. I bought the car second hand about 30 000km back (it now has 170 000 km), and assume the previous owner has not changed it.

So, I asked the Merc garage to change the fluid when I went for the service. But they were reluctant, saying in the past they had problems with diffs singing/whining after fluid change.

I dont want to scratch were it is not itching. Its purely preventative maintanance. What is the chances of this happening?