From what I was told your vehicle car or truck should not bounce or rebound more then one and a half times over any bump. A shock dampens or controls the bounce, sway and dive during braking.
By the way a light oil film at the top near the piston is not a sign of a leak! If you have oil and dirt along the tube then you have a piston seal failure and your shocks should be replaced.
Steve with HD 250/2500 and up trucks it is hard to tell as the springs on these trucks are stiff. I use the 50,000 mile rule and just replace them there, unless I see oil/dirt all over the shock or strut.
Don