It's important to understand the purpose of a "shock absorber" and what part does what.
The key thing to note is that the oil doesn't actually absorb the 'shock' of hitting a bump, the spring does. The oil controls the oscillation of the spring - without the oil the spring could continue to compress/decompress long after actually hitting the bump.
The purpose of the shock absorber as a unit is two fold - to absorb bumps in the surface, and to control the sway of the chassis as you corner. So, it controls the speed of the weight transfer, and how much weight proportionately is applied to the wheel....as well as the afore mentioned dampening properties.
Think about it conversely. If you run your shocks empty with stiff springs, the car will skid all over the place and skip over bumps. If you run as you are with heavy oil and light springs, the spring is being overpowered by the oil so you have too much of a dampening effect occurring on the spring. Therefore, it won't be able to absorb shocks, or react fast enough to bumps and dips (because the spring is being prevented from keeping the wheel in contact with the ground) - or straighten the chassis out after a corner efficiently. This will often present itself as poor handling (pushing on, spinning, squirrely behaviour after a corner etc). Generally, a stiffer shock will provide better cornering grip, at the expense of fast reacting shock absorption (or 'bump') properties.
The key is to get it right for your circumstances. If you are bottoming out hard after a jump, stiffen up. If you are bouncing over bumps, soften up.
Don't forget that how the shocks are setup plays a huge part in handling, as does the angle of the shock. For example, if you're too soft in the front, you could make the chassis dive, pick up a wheel going into the corner and spin the back end out, or roll. Similar story if your rear shocks are too upright or too soft - so step back, think about the problem, and the root cause.
Best advice I'll give is look at a) what others are doing and b) team setup sheets. Go back to basics, start with the kit setup and make minor adjustments one step at a time.
I don't know what car you have but if it's a Schumacher SX try 4 hole pistons all round, 35 in front, 30 in rear, with the kit springs, and adjust from there. Note that the spring tensioners (ie, the shock collars) do not adjust weight of the spring, only ride height.
I'm sure there are much better mechanics/engineers/drivers out there who will give you better advice, I just wanted to throw my 2p's worth in!
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