Here's an unusual HP. This is a heat-sink that I flipped over. This particular HP fan sucks though the heat-sink rather than blowing into it.

The grayish material is a heat transferring compound that contacts the CPU. Long story.

Anyhow, you can see that an owner was a smoker. This linty material would not remove cleanly with compressed air and I had to physically brush it off. Then I took it outside and sprayed it with carburetor/throttle body cleaner. The brownish sticky gunk just rolled off for quite a while.