Wow...this thread is certainly a mess of discussion.
Shock oils/silicones and their weights. The differing weight of oil/silicone is all about the viscosity of the oil/silicone. All else being equal between two shocks, if you have a heavier oil in one than the other the one with the heavier weight will rebound much more slowly than the lighter weight.
Pistons and the number of holes in them. The different number of holes in the piston head are there for a similar reason as the various oil/silicone weights. All else being equal between two shocks, the one with fewer holes in the piston head will rebound more slowly than the one with more holes. Think about moving you hand through water. If you have your hand out flat like a paddle and move it through the water, you feel some resistance. The faster you move your hand the more resistance. Now spread your fingers and do the same thing. The resistance you feel is lessened. Same concept.
That about covers the dampener portion of the shock.
The springs are a different animal and serve one purpose (maybe two). The one purpose is to rebound the shock. The dampener is there to absorb the shock. If you use a really light weight oil and a really strong spring, you now remove the dampener as the absorber and make the spring do all the work...or most of it. The second job (the maybe one...) is that the spring can also be set up to shoulder some of the shock absorption.
Different weight springs do different things. Most people who go for big air or have heavy rides, use heavy duty springs. Most people that do a lot of offroad racing on rough terrain or groomed offroad tracks use lighter springs coupled with lighter oils to allow for rapid articulation of the suspension. The bottom line is that there are as many set ups out there as there are hobbyists.
Dual rate springs versus single rate springs. Single rate springs maintain an average compression rate throughout. In other words the force that they exert is close to constant throughout their compression. Dual rate springs work differently in that they tend to increase the force that they exert throughout their compression. The more you compress it, the more it resists.
That being said, pre-load spacers on Single-rate shocks do nothing more than increase the ride height by making the spring try to decompress thus pushing the truck higher. Will it cause the spring to rebound any quicker? No. It may appear to do so but in reality it does not. Simply put, the spring will exert constant force in resistance to the compression. This in turn pushes the ride up as far as the spring retainers will allow the spring to de-compress. On a dual rate spring, you may see some change as the force is different depending on how far you compress the spring.
So, what does changing the oil, piston head, spacers, and springs do? That is up to you and what you hope to accomplish with your ride. Since this thread is in the T-Maxx Talk section, I will go out on a limb and say that you are running a Maxx of one variety or another (T-Maxx, SportMaxx, or some hopped up variant of either). If all you are doing is running around on the block or playing on relatively easy terrain (which is what you have described,) you should be fine with 30-50wt oil, two to three holes in the piston head, as much preload as you desire, and I would recommend some Trinity Blue springs at all corners. This will give you good suspension movement, decent rebound time, and some strength should you get adventurous and take some air from something higher than the curb.
Hope this lengthy post helps.