I have an old p4 3.06 system I salvaged and it was overheating so I removed old thermal paste but don't have any new stuff around so I will try toothpaste and see if it makes a difference, bear in mind this thing with an 80mm CPU fan and no paste is in BIOS at about 100C so I will try it and report back if I get the chance.
Yeah, I'm gonna say no. How many years will those substances keep their thermal transfer properties? I have have the same AS paste on my CPU for at least 5 years.