Heavier fluid increases damping, rebound damping in particular.
Increasing fork fluid volume increases preload much like air would, but you have to be very careful as you can increase it only so much and a little goes a long way.
The stock fluid is a 10W equivalent. For me 15W worked fine but some heavier people might do better with 20W.
I increased volume by 1.5 OZ and never used air.
On the 02 RK you would have to modify the fork caps to the ones from an air suspension just to allow rapid fluid changes. The RK's do not have the cartridge in the left leg, they are just conventional forks.