You can use
any fork oil in Fiona... Different weights will yield different performances: the higher the weight, the stiffer the forks. The book specifies
Dexron III ATF, but that is because it is uber-stable in viscosity over a wide temperature range (just like legitimate fork oil), and it is supposedly around 10W equivalent.
I installed
Progressive Fork Springs, and
Rear Shocks, on my '79 K model: rides like a modern, 21st Century bike! Worth the investment, IMO. It is more than just comfort, though, as it tracks the road surface much more accurately, which means greater safety, and control. It is a bit of cash, but worthwhile, if you put the miles on the old girl. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII