I use RamAircraft's suggestion for slip joints and it has worked well for many years on various aircraft.  Ram recommends coating the contact surfaces with Never-Seez although I found that the Permatex Nickel Anti-Seize high temp paste is less expensive and works perfectly on the Lancair exhaust.

 

Apply whenever the exhaust is apart.  It will help seal small leaks, allows parts to move during operation and I find this prevents a lot of leaks.  It also prevents engine vibration being transferred to airframe.  Since applying the Permatex to the Legacy, all the small leaks are gone and the slip joint are visibly sliding--before they were stuck.