I have had the Bilstein 4600 on both my 2500HD and 3500 for 10 and 5 years respectively . The fronts developed a problem with the bottom rubber mounting bushing . I notified Bilstein and they sent me 4 new front shocks . The new shocks have a redesigned bushing mount .
Being curious I clamped one end in the vise and tried pumping up an down . The old shocks felt identical to the new ones . I was impressed . No sign of leaks . The 2500HD had 130k on them and the 3500 had 40k with a 1k plow hanging off the front for 5 months a year .It is literally ridden hard and put away wet .
As a reference , the OEM shocks on the 2500HD had less than 20k when removed . You could move them up and down with 2 fingers .
Just one mans experience .