Madmark,
a lighter shock oil gives less damping and allows the wheels to travel up and down more quickly. the opposite is true for heavier oils. But wait theres more. you also have to take into consideration the hole size and number of holes in the piston(plate that attaches to the rod inside shock body). More/larger holes act like lighter oil. Also the !QUOT!weight!QUOT! of your springs. Heavier weight rating of the springs will slow down the compression and speed up the rebound. hope this helps.