A job I will need to do soon as I am now a couple of thousand miles in to my replacement headers. Some good advice on here.
I prefer to use Plusgas as a penetrant on seized fasteners instead of WD40 as it penetrates with a very thin film and high surface tension compared to WD (which is more of a general purpose goo for displacing water, corrosion protection by surface film formation, and making plastics clean and shiny.)
Graphite powder or high nickel content paste would be my weapon of choice for lubricating a hot exhaust valve (unless I can find an old tin of Clydespin at work).