Almost every Mercedes in recent history has a power steering filter. Check with your dealer or supplier, such as FastLane.
There have been multiple discussions on this forum regarding which fluid to use in power steering units. Some of the more respected members advise against using ATF. According to them, yes, it is a hydraulic fluid but it has additives that are not beneficial to your power steering system.
They recommend either the Mercedes power steering fluid or regular everyday over-the-counter variety.
I believe if you do a search using: Larry Bible and Power Steering you will find his numerous comments.
It would be a good time when replacing the filter to also flush the system until the fluid returns close to its original condition and color.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)