Most ATF oils are around 12-15 weight and are very acceptable as fork oil. If you want to go lighter the Mobil 1 synthetic ATF is around 7.5 weight. The ATF work fine as the fork seals are very similar to transmission seals. You can use standard motor oil in a pinch but I would only do so if I had bad leaks.
Personally I use synthetic 10wt fork oil made by Rock Oil.
Fork oil viscosity doesn't do much for diving under heavy braking, because the weight transfer lasts long enough for the most any weight oil to flow through the fork dampers. . Heavier weight oil makes for a harder ride, lighter oil for a softer response. Stronger springs will reduce diving somewhat at the expense of a harder ride. I've had Progressive springs in three of my 'Wings, an 1100, 1200 and my 1500. I like the improved handling especially in the twisties.
Diving control is available in the later model 'Wings but the early models didn't have any way of adjusting dive response.
i have 2 spring's in my gl1100 one long one and 1 short one
lot's of tension, threw my back out 2 yrs trying to change oil with them on the bike ended up in hospial for 2 day's
this time i took them of the bike so much easier. and i'm still walking around no pain....
When i changed the juice on my '82, I wen't to the auto parts store and grabbed the $1.99 qt of Mercon 2 I believe it was.Drained out the old and in with the new.It's not rocket science, but E still ='s mc2.Or is that cubed?.,,,,
Anything is better than the factory fish oil. Same with those springs. Progressive has been around for years and for around $50 does a great job on the GL springs. I'd also change the headset bearing while the fork tubes are off.