tube damper help

I use 3 mm white Marigo VTS dots under all of my tubes.

'black' may be OK for low power tubes. Black may be Viton, Buna 'n' and many others, including just plain rubber.(neoprene?)
Silicon (red?) has silicon oil in it which can migrate out and make a real mess, on higher powered tubes. If i had tubes and thought damping would help, and it may in certain feedback situations, I'd use EP, which will take more heat, is inert to many (certainly around the house) chemicals and is not too soft. EP stands for Ethylene Propolene.
If all you care about is maximum hi-temp use, try Kalrez. Good luck finding this at the local emporium, but I think that 500f should be good enough for most power tubes, too.
http://www.mcnallyinstitute.com/04-html/4-9.html

I use my own recipe. See my thread here

Brass and high temp rubber. You can find small ones at Brent Jesse "audiotubes.com". I have the something similar in a discontinued 6SN7GT size which are very effective. I don't know where you can get them now.

Magfan,
I've been using silicon "o" rings on my KT-88's for over a year have had seen nothing like you describe.They barely get hot to the touch.Maybe we're talking about different types of silicon,who knows?