Some thermal grease deteriorates over time and can result in poor conduction of heat, which could ultimately burn up your CPU. The top brands of thermal compounds like Arctic Silver and others do not generally suffer these problems but AMD has no way to test every thermal compound sold Worldwide so they prefer to error on the safe side and use TIM for the XP series CPU's which have very small CPU dies and need good thermal transfer to prevent overheating.
Everyone has there favorite thermal compound but based on any number of tests Arctic Silvers products are some of if not the top thermal compounds available. I personally use the Ceramique from Arctic Silver as it is non-conductive and can't short circuit electrical contacts like some other compounds. In addition Ceramique is about as good as you can get for thermal transfer too and it will not deteriorate over time. I would recommend you switch to Ceramique if you can in the near future. That doesn't mean there is a panic, but most low cost thermal compounds sold at PC stores, Radio Shack and elsewhere are for general use, and are less than ideal for a current day performance PC system CPU application.
The Arctic Silver website has all the product details and important installation instructions for proper use of their products. In the case of thermal compounds you want the thinnest possible layer of compound so it just fills any surface imperfections in the heatsink and CPU die. See the Arctic Silver website for good tech info. on proper installation as excess thermal compound is a real no-no on CPUs.Arctic Silver Website
If only they knew...