Really quick search shows average BP temps to ignite between 500 f - 860 f with a combustion temp of just north of 2550 f
Better get the jet lube 
Kevin
Anything that's decent is better than nothing. Breech threads are hidden from direct contact with ignition, so a temp range is really all about how long the lube/grease will last before getting cooked out.
For me, this thread lubing/greasing concern is about ease of removal and something is way better than the nothing.
With all the Kibler breech plugs I've removed so far - a dozen - I never detected any lube/grease. To be fair, they were all relatively easy to remove with proper tools. Which makes sense since these bbls and plugs were never fired, ever. Jim's remarks in forum posts concerning the anti-seize lubing/greasing of breech plugs, or anything screwed into a BP bbl, is mostly on the order of it doesn't matter. I think that's a long term mistake. There is NO need to remove breech plugs for any cleaning/maintenance reason. But there are some RARE instances when it is desirable, such as installing a new touch hole liner, or a stuck ball/load that won't respond to normal methods, or to lap the bore, or to "fresh" the bore. For a few bucks cost of any quality anti-seize lube/grease, if ya can do it, why not?
The offshore gun breech plugs are hit or miss for more than a few reasons. None are lubed at all, for the few dozen I've pulled off over the decades. Some that were in brandy new guns were near impossible to remove; it was like they were welded on. Here's some text from the Traditions manual for their traditional side lock guns ...
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... interesting, eh?