Increasing the spring rate will increase the effect of compression damping.
So if you are searching for a particular setup, you may not want to increase spring rate AND use thicker oil straight up. Do one, ride it a few times, then do the other. You may end up draining the oil out and going back to the standard grade. It depends on where/how you ride, and exactly what sort of springs you got for your weight.
To make sure you understand WHAT changed in setup, you should only make one change at a time and then give it a good evaluation. Preferably over the same ground at the same speeds.