The spring is there to counteract the weight of the saw, so you adjust it so the saw doesn't put it's full weight on the blade/workpiece. You can try different settings but around 5 to 7 pounds downforce should cut pretty well with a good blade
The weakness of the spring device is that it's not linear
I use oil when cutting steel, messy but makes the blade last longer
The factory blade is pretty horrible I have heard, replace it asap
-M