"The thread suggests that you can use STP oil treatment but from what I remember of my engine building days, that stuff won't come off whatever it touches."

It was approximately a month ago when I started the thread and used that STP treatment. The temp hasn't risen past 100 degrees in traffic ever since then. All with stop-and-go traffic and AC on. Before it used to happily climb to 120.

The clutch isn't seized either - it rotates relatively freely. Of course the question is what that STP does in a year's time but should it fail somehow I can always inject some new in to the clutch. Also it could be wiped off with a piece of cloth quite easily.