Kitty-litter is usually clay (there are many other kinds though). That's pretty good at adsorbing oil (when spread onto a spot, and ground in a bit with your shoe).
Sweeping compound is usually mostly sawdust, with either an oil, or water based liquid added (and usually a green dye). This won't pick up oil spots. Where it excels, is in picking up dust. Sprinkle it all around (just a little bit, all over), and as you sweep, it sticks to the dust, letting you pick up more dust with a broom, and keeping the dust out of the air. By the time you've pushed the green compound in front of your broom all around the shop, it won't be green any more as you pick it up in the dust pan.
It's the shape of the sawdust (it's kind of "fluffy", with a large external surface area) that allows it to stick to the fine dust. Clay has a fine internal surface area that allows it to wick oil. You can't interchange the two.
The oil based ones don't dry out, but are flammable.
A 50lb bag shouldn't be more than $10 at a real hardware store (my local Ace carries the water based one, so I keep it in a sealed bag).