When I rebuilt my hot section this week I used
MASTERS® METALLIC COMPOUND by G. F. Thompson Co. Ltd., Newmarket, Ontario, Canada.
It's rated for 550 degrees F and 6000psi, unofficially its been used for steam way past 550.
I also used it as never-seize/lock-tite on my cradle bolts today, the nut won't free spin off and will undo next spring.
Last year the can cost ~$40cdn and should last ~50 years at my rate of use.
Bruce