I'm running Progressive Springs. Huge improvement over the stock. I think the Progressive Springs were more of an improvement that the Progressive Shocks. But then I don't often have a passenger.

As far as changing the fork oil. You will need a special fork nut socket. They are around $18. I think I found mine on ebay. You will also need a motorcycle jack, or some other way to get the front wheel off the ground. I also recommend a palm wratchet it's easier to get the spring compressed and the cap nut back on. I think the custom models are probably easier. With the Low model, you have to compress the spring about 6 - 7 inches to get the cap nut back on. That's with the stock springs. The Progressive springs don't require nearly that much compression to install.

I don't think I would attempt this without a service manual.

 

Opinion noted.
But the bottom line is that the sag is happening because the stock springs have become weak. Heavier weight oil in this case, is only a bandage on the wound. Just my .02 for what little that's worth.

 

I have the Progressives on my 1200C (2006)
Went slightly longer on the pre load spacer's to 3.200 inches and used Amsoil 10 wt shock therapy fluid.
We did add a little extra oil,about 1.5 ounces extra in each tube (just a little under 12 oz)
For my 180 lbs it feels really nice
Now if i cant some slightly lighter springs for my 418 rear shocks i will be set !

 

I too replaced the stock fork springs with Progressives and run 7w BelRay, no bottoming at all. I used the spacers at their full length. Its nice to feel the forks actually working. Progressive 13.5" 440s on the rear. My riding buds say my taillight no longer bobs up and down on bumps. I know that bumps are now a non event from my spines point of view.

 

How easy is it to change out the fork oil? Id try that first...