I just replaced the motor mounts in my 300E. They do not appear to be fluid filled. The rubber in the old mounts had compressed. Before I changed them, they seemed loose, the bolts took 4 or more turns easy. New mounts were not very expensive (PartsShop.com) and only took an hour to install. Put the car on ramps and make sure you have a floor jack handy. Very straightforward. 300Es are a dream to work on. Better than therapy.
'88 300E 265K
'91 300TE 155K