Qman. You need to get either a Honda Service manual or something like a Clymers manual. Questions like this are answered in the books.
My Clymers book says SS7 which is the equivalent of 5w. I use Bel-Ray in all of my bikes even my vintage stuff. I have never used AFT and I have read threads that say that the AFT is not compatible and the seals will fail sooner. I have no proof of this.
Our ST1100 have a generalized dampening system meaning that they work okay across all road surfaces. Depending on the ride you are after the oil weight may impact the low speed ride or the high speed ride. The orifices in the dampening rods are designed for the fork oil weight recommended by Honda. In my opinion I highly doubt that the orifices sizes will enlarge based on oil flow alone. Although I have not checked this the logic does not make sense unless one has hundreds of thousands of miles on the bike. There needs to be some other abrasive in the oil to make this happen.
Two things will affect your dampening, oil weight and air volume in the fork cavity. Oil is not compressible the more oil the less dampening. Springs do not impact dampening. I believe Bel-Rey come in like 5, 7, 10, 12, 15, 20, etc.
I run Bel-Rey 5w oil and have 1 KG straight rate Racetech springs. I did try 10w with these springs and found the dampening to be very rough at highway speeds. Back to 5w.
If you are interested in suspension get the Racetech Suspension Bible.
Here is an interesting article.
http://racetech.com/html_files/DampingRodForks.HTML