Springs are the basis of the suspension action. They let the bike bounce. The spring rate determines how much force it takes to compress the suspension, and therefore how much the suspension moves. If you have the wrong spring rate, the suspension simply won't move the right amount.
The oil system acts as the damper for the suspension. The springs convert motion into energy, and the damping system dissipates that energy. Without damping, the bike would just continue to bounce back and forth forever (ignoring friction and other imperfections of the real world). If your springs are a tad weak, some extra damping can help slow down the excessive movement. But if your springs are way off, trying to fix it by changing oil details is asking the system to do something it's not intended for.
The NewGen and 300 have the stiffest
fork springs of the little Ninjas. They're good for 86±10lb. That sounds pretty weak, but going up two notches is good for ~150lb, and three is good for 180lb. For comparison, the PreGen springs are about 2/3 as stiff, and three notches up is only good for ~110lb. The NewGen and 300 are soft, but not horribly so like the PreGen and 500.
For a bigger guy like me, changing my 500's springs to match my weight was the best mod I've done. The suspension actually moves like it's supposed to now - it properly absorbs bumps for a smoother ride, while supporting my weight in corners and such for a more planted feeling. Too-soft springs will make the ride both mushy and rough at the same time, as illogical as that seems. But if you're smaller and have a bike with less-bad springs, the stockers might not be too bad for you compared to the cost and work of swapping in springs with a perfect rate.