It isnt, just dont pack the hole---
*The warranty is void if you use anything other then the TIM supplied on the heatsink included with a retail chip---do they remove the heatspreader and look for AS3 packed in that hole upon RMA? Or check it some other high-tech way---who knows ?
(if its OEM, do what ya please I guess---dip it in guacamole YUM!)
--->Oh forgot to add---ive removed the TIM off the dozen or so ive installed and used AS3...