I also have a 2008 XLC. IIRC the owners manual and the Service Manual incorrectly listed the torque values as way too high. Some people using the incorrect TQ nos. cracked the cases. Be careful. I continue to use/reuse the original "rubbery" one without problems. I have changed the primary fluid and adjusted the chain 4 times. I did replace the small o-rings and just snugged the screws. No leaks whatsoever.

 

Maybe try an auto parts supply for a similar "O" ring to your original, they look pretty standard to me.

John