Hi Nigel
The post by Spitfire shows the capacity as 160cc which in my Clymer manual says should be 10W30 for the CB550K0,K1,K2 but the K3 forks were revised and I think that for them it should 190cc and ATF.
I have used ATF in all my bikes which gives a ride that suits me , however you may want to go for medium grade fork oil of a good make to firm up your forks - you mention that it has too much give. Wemoto is a good bet for well priced fork oil and low postage. ATF is kind to seals and cheap, it seems to have disappeared from my car shop and replaced by AQF which they say is the same - anyone else have any ideas if it is the same as id love to know for sure as I am just about to fill my 750 DOHC forks with it.
Here's a pic of my K3
Kind regards
Hubert