Yes, toothpaste can be used, I did so on my old core 2 duo E 6750 for about 2 years and even overclocked it abit. Worked like a charm.. But i must say after few months of use, it dries up, It might still keep the processor in contact with heatsink but its pretty hard to scrap off the dried paste from the processor surface... I changed my toothpaste yearly...
If you are up for an experiment,,, then best of luck.!!!