Hi everyone, URO Engine Mount 951 375 042 04 is indeed liquid-filled, which you can verify by viewing the check-valve ball located in the center of the mount opposite the stud. It looks like the end of a grease zirk, and the propylene glycol vibration-damping liquid is injected through this port during the manufacturing process. Hydraulic mounts are more difficult and costly to manufacture, which is why many aftermarket mounts are solid.
Failure of hydromounts is usually due to the eventual cracking of the rubber due to flex cycles, heat, age, and exposure to ozone. Once a crack breaches the inner wall of the mount, the damping fluid quickly leaks out and the mount must be replaced. Hydromounts have a shorter lifespan than solid mounts, but the reduction in vibration and harshness is worth the additional maintenance for passenger vehicles where comfort is a high priority.
Underhood Service magazine has a good article on the subject:
Motor Mount Logic
This mount is covered by URO's two-year warranty, and is available from Pelican here:
https://www.pelicanparts.com/More_Info/95137504204M253.htm?pn=951-375-042-04-M253&bt=Y&fs=0&SVSVSI=3306
