ORIGINAL: Doahh
The thicker you shock fluid is, the slower it will roll (body roll that is) and the less initial grip it will have. The stiffer the springs, the less total roll, and in theory less initial turn in.
The thinner and softer, the opposite.
If your car needs more grip at either end, you should try softening up the suspension at that end first (assuming the suspension has shock oil in it, cos shocks can be leaky, and if theres none, well use your imagination) by using softer springs , thinner oil, or larger/more holes in the shock piston.
For the sake of accuracy, I must debate these points here too.
The thicker the shock oil, it will have the same turn in as it would with softer oil, but then immediately after the truck will plow. So heavier oil affects mid-turn, not turn in. Also heavier springs will cause your truck to have more turn in as well. Softer springs will cause the roll rate to increase so the truck reaches mid-turn quicker but at a trade-off at initial turn in.
Also, if your truck needs more grip at one end, it isn't always wise to just soften it up. It is important to look at the handling properties of the entire truck. If your truck is already soft and one end does not have grip, that means the other end has too much grip, so the other end is too soft, so stiffen the opposite end to bring the truck to neutral steer and achieve balance.