Recently I changed fork oil from 10W to 7,5W (Shell Advance) which is about ATF weight. ATF was what Honda prescribed back then. By comparing partsnumbers I discovered that my model CB500 has in fact the same fork legs as the CB550K2, so instead of 160 cc I filled them with 145 cc.
As long as you don't operate your motor around zero degrees, 15W-40 is maybe a better choice than the all temperature 10W-40. Honda France prescribed even 20W-40.