Quote from my local comp shop techie:
"Using anything other than the supplied thermal pad voids the warranty. That being said, they don't KNOW that you used AS5 if you clean it properly. So as long as the CPU has some remnants of the stock thermal pad on it, as far as I'm concerned, the warranty is still good. Just don't TELL me you used AS5.*wink wink, nudge nudge".
i.e., clean it, run your comp for a few hours with the stock HS+original thermal pad on it, and all is good.
Which is why I always keep the stock HSF and thermal pad locked up in case something happens.