I use Arctic Silver 5 on mine but if getting conductive paste on those circuits can actually cause problems (which we don't know; they could be coated or something), you need to be extra careful if you use any metal or water-based paste.

And it turns out the AS5 is not actually electrically conductive. The oil suspension they use must be very good as an insulator.

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Not Electrically Conductive:
Arctic Silver 5 was formulated to conduct heat, not electricity.
(While much safer than electrically conductive silver and copper greases, Arctic Silver 5 should be kept away from electrical traces, pins, and leads. While it is not electrically conductive, the compound is very slightly capacitive and could potentially cause problems if it bridges two close-proximity electrical paths.)

Didn't know that. I'm not suggesting AS5 in particular, but it's one of many good pastes out there.