Assume that you have two shocks that have very similar static dampening. One has pistons with large holes and thick oil, the other has small pistons and thin oil.
The shock with the small pistons and the thin oil will have more "pack" than the other shock. "Pack" is a term used to identfy the shock's initial reaction to sudden and sharp compressions.
Pistons have more effect on the "pack" of the shock, while oil mainly concerns the entire stroke of the shock. You will need to use both to fine tune your car's handling.
Aaron Waldron - LiveRC