There are special lubes made for chokes or you can use a standard anti-seize.
Most gun shops will have one or two, or you can order from Brownell's or Midway.
Clean the threads of the tubes and the barrel by using a bore solvent and a brass "toothbrush" to scrub them.
For extra tube, check with EAA, the current importer:
http://eaacorp.com/blog/2010/10/29/b...-back-at-ussg/