I use Permatex Red or Copper RTV and have had outstanding success with it. So much so, I no longer use header gaskets at the heads, just a thin bead of this RTV (even on race cars). No leaks, no problems, o2 sensor safe.
If the slip joints are good you can use grease for assy and disassy, makes it easier but grease is not a sealant. If the joints are sloppy use any 100% silicon sealer, you know the stuff they make hot pads and trivets out of. Can be kind of tuff to separate though.