one thing that people should keep in mind is that kitty litter often comes from different sources, depending on what is cheapest for the manufacturer to acquire and ship. as kitty litter, it doesn't have to be free of dust or hold up very well when wet, since it is expected that you will only use it for a cat to do its business in. oil-dry on the other hand, needs to be as free of small particles as possible. the large clay pieces have to hold up so that they can absorb oil without turning to mush. otherwise, you wouldn't be able to sweep it up. this is a big part of the reason that oil dry generally receives higher ratings, its more consistent and is designed to NOT fall apart easily. having worked in a transmission shop when i was younger, i can tell you that kitty litter is often a pain to work with when cleaning up larger oil spills because of its properties. makes sense, its made for a different purpose.
some plants actually prefer the "mud" that some of the softer kitty litters produce, but very few aquarists are willing to deal with the hassle it causes. personally, i have a mix of special kitty cat litter, aragonite, and cow manure. it works pretty well for me, better than any substrate i have tried in the past.
the point? kitty litter may work, but you cant be sure unless you know where the manufacture got it from, while oil dry will consistently be the hard type of baked clay most people prefer.
as for me, i like the mud that the kitty litter makes, so im going to stick with it.