i have an s2 fz-n ive done the hyperpro fork spring mod and supplied 15wt oil and for track days there great i can really load the front,however on normal road riding at lower speeds the front feels a little hard,the bike reacts over small bumps instead of absorbing them, i would like it a little softer but dont know if i should shorten the preload tubes or use a lighter oil say a 10wt
I'd guess you're headed in the right direction by draining the 15wt in favor of 10wt. Think of it this way; the spring dictates how much load you can carry so check the sag and ride height.
The dampening controls what you feel. You only have control over compression dampening so if the ride height is good, you can reduce the harshness by lowering the oils viscosity.
Also keep in mind that you can mix weights if you feel there is a need (10wt & 15wt for example).
How much travel are you using as the norm on a street ride keeping in mind that if the dampening is to great the suspension won't collapse so it CAN BE misleading. Dampening takes time and small bumps may be transmitted to you instead of being absorbed if the dampening is too aggressive. Does that make sense?