normally a tiny portion on the center of the surface is enough.

as for the AS5 instruction, IMO AS5 requires more care and careful application because it contains silver and copper compounds which can conduct electricity and may end up causing damage should it come into contact with parts like electrical traces, pins and leads.

please please please don't apply paste like jam. when you put the heatsink on, it will automatically spread. the purpose is to aid in surface contact. that's it.

you put it think or in huge quantity, it becomes messy.