I have read some discussion on changing the oil in the rear shocks.What would be the purpose in it? How often should it be changed? Should it be changed? Why one weight of oil over the other? What is to be gained?

 

Replacing with a heavier weight oil will increase dampning (slow down compression) and slow down rebound. A lighter oil will have reverse effect.

 

What would you recommend over stock oil? This being the case,then you would be able to lower the air pressure. Sound about right?

 

Yes.

I changed mine using the method on this forum ( https://www.hdforums.com/forum/touri...ur-shocks.html ) and went to SE fork oil that is about 7wt. IMO the shocks work better and I now run a lower pressure . I ride solo most of the time and now run between 8-10lbs air. What I notice now is the shocks actually seem to have better dampening and are smoother, the down side is big bumps / pot holes will make them bottom with the low pressures but 90% of my riding the shocks are now able to work properly and use their full travel. So overall the ride is much better

 

Think of your air pressure as if it is your spring on your shock. A spring/air pressures only function is to negate gravity. It's the only thing that holds your bike up. The combination of your spring/air pressure in combination with the dampning valves along with the oil viscosity all determine how your bike reacts to small and big bumps. The trick is to find the combination of all three components that allows the bike to absorb the small bumps yet when you hit a big bump you don't bottom out the shock and thus feel it up in your lower back.

The big advantage are shocks that give one the ability to adjust air pressure or spring preload based on whether we are riding solo or switching to riding 2up for the day. Riding 2up you're going to want to increase that airpressure/spring preload.

 

I replaced the rear shock oil last summer on my 2010 RK with Amsoil Shock Therapy 10 Wt. Riding solo I'm using 10 PSI. The ride is a little firmer and don't bottom out. I know guy's that are not using any air pressure riding solo. I tried that and got oil up in my air lines, so I use 10 PSI. Works for me!!

 

ATF - about 7.5wt. Worked great, i was surprised. Its an inexpensive mod that can drastically change your ride

 

There are a bunch of hydraulic oils that are suitable. ISO 15 is roughly comparable to 5 wt and ISO 32 compares to most brands of 10 wt shock oils. ATF tends to run between these.
There doesn't seem to be a rigid standard for determining an SAE wt. rating for shock oils so there will be a variance between brands just as there is with most lubricants. There are a couple of charts floating around that have the viscosity and other details listed for most brands.
An important detail when changing shock fluids is the amount of oil they end up with. Even a small difference changes the space available for air.
Less air space makes an air pressure change more pronounced.
As long as the shock can be fully compressed (with air lines disconnected) and oil doesn't come out, then IMO they aren't overfilled.
Captsluggo