If you have rebound that is "too slow", a lighter oil is what you are looking for.
A thicker oil will have a more difficult time traveling through the valving, and slow rebound.
Im not sure what specific fork oil we are supposed to have. If its showa ss7, and it may be, that looks like its a 5w.
This chart works really well as it lists so many of the fluids we have access to. You can see, also , that someones 5w may be different than others 5w. They break the fluids down and measure in cSt centiStokes. Suspension Fluid - Pvdwiki
Just a fancy name for soemthing more accurate than 5w, 10w, etc. The lower the number, the thinner the fluid is.
From the list, the redline (like water, clear) fluid is probably the easiest to get. My local shop sells it.
Lets say you dumped out the stock oil, then switched to this redline, it should lighten the rebound. I say SHOULD because as chiffeee said, the stock oil is like water, anyway.
The bad part about messing with oil weight is it also messes with compression damping. It will make that lighter, as well.
As a compromise, I think I might try this.
Wanting to decrease rebound makes me think the fork is harsh. Hits a bump and does not ready itsle , fast enough, for the next bump.The stock Kawasaki forks get pretty harsh, halfway through their travel.
How about increasing preload, a few turns. Bring the fork back up to a position where its not running into the middle of its travel so easily.
At that point, the bike may be too tall. Bring the fork up through the top triple clamp 5mm so the front end stays about where it was.
The cartridge forks are a pita to deal with, in regards to oil. Unless you get 100% of the oil oil out, you'll end up with some odd mixture of stock and aftermarket fluid. Even if it were amazing, you would never be able to duplicate it again if you blew a seal, or needed fork service.