When it comes to sealants and adhesives I know almost nothing. My head starts spinning when I walk in that aisle because there's to many options.
Man, isn't that the truth, you damn near have to be a chemist when shopping sealants.
Any search engine like Google is your friend, I searched 'best high temperature rtv' and got this on the first page. Just pick a known manufacturer like Loctite and you'll be safe, this stuff has been perfected forever. Don't apply it to smooth plastic, scuff it up a bit for traction.
"When you need high heat resistance,
red RTV is the ticket. Any high-heat application should get red RTV, including exhaust manifolds, drive housings, even oven doors. Ultimate Red from JB Weld is sensor safe, so it can be used on O2 sensors. Red RTV is good up to 650 degrees Fahrenheit."
I was surprised at the 650* rating, many epoxies are only good up to 200* and 5356 aluminum welding wire is only recommended up to 500*.
All this was from searching for it. As a #oleguy, it really made my life easier when I started going right to 'search' when I have questions.