The FSM calls for 160-165cc of ATF, which is pretty much 10W. I used ATF when I rebuilt the front shocks on my '82 CB450SC and had zero trouble.
Now as for what is the best there is a lot of room for discussion. I know that oil weight can have a big impact on compression and rebound damping. I've got no experience with dedicated fork oils like BelRay or Maxima but someone should be along in no time to expound on them.
HamHock is correct on the oil weight effect on damping. Its pretty much on how stiff you want the shocks. Just for info when I rebuilt the shocks on my CB 77 the guys on the 305 site use the same oil they put in their motors. I have since sold the bike but have not had any complaints.
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