You don't want it to be heat resistant. You want it to conduct heat
as much as possible, so that the heat can flow easily away from the
LED and into the heat sink.
This is a common, everyday situation in electronics construction,
and there's a special substance formulated for exactly that job.
I don't know what its real name is ... I just call it heat-sink-compound,
-paste, -grease, or -goopy. It's available everywhere, and I know
for a fact that it's available at the chain of stores which I can't name
here but which rhymes with Shady-o-Flack. I've bought it there.
I'll even give you a tip on how to handle it: It's not dangerous, but
once it gets into anything or on anything, it doesn't come out or off.
So when you're working with it, wear disposable gloves and a bib.