2010 and 2012 C350 are they using the same Power Steering Fluid.
When I need to change them and some say is life time just top it up if need ( I really don't believe is life time) and under the manual it say Chevron Texaco PSF 9109 and it look like it hard to find. So site say Mercedes 2008 - 2010 they change from CHF 11S to Chevron Texaco PSF 9109. But dealer still selling CHF 11S. Is Pentosin CHF 11Sok to use
Since there is no indication of the steering fluid interval replaced in the maintenance booklet, that means no need to replace it.
But if you'd like to replace it, try to find the fluid with MB Approval. Here is the link: https://operatingfluids.mercedes-benz.com/sheet/345.0/en