Also just used toothpaste on my macbook pro 2011. It had a failing graphics card and being out of warranty I waited until it was truly dead => no boot.
I took it apart, cleaned it and removed all plastic. Did a "reflow" of the motherboard in our regular oven at 220 degrees Celsius for a couple of minutes. Applied toothpaste to the cpu and gpu and put it all back together.
After all this it booted right back up. It ran hot for 2 minutes at 90 degrees Celsius. (might have been a diagnostic check after being taken apart, OSX just testing the insides)
It now runs at 45 degrees Celsius under normal load. No crashes, no funky broken graphics cards stuff anymore.
It will probably set itself on fire at some point, but more likely because of the failing battery then because of the toothpaste.
Just to be clear: this is an old machine that has been abused for many years.
I work as a photographer and this is the machine that travelled with me. So it saw lot's of water, dust, sand, mud and everything you don't want around electronics.
Do not try this just because you are too lazy to get the real stuff.
Only badasses may proceed!
The gpu finally truly died. Well not really, the gpu is perfectly fine, even after months of having toothpaste for thermal paste. The solder connecting it to the motherboard however has too many cracks in it now and a reflow in a cooking oven no longer works.