honestly, the cat litter is a good idea. once i created an oil stain on
a customer's driveway and i tried expensive concrete cleaners, etc.
Finally what worked(to an acceptable level, nothing is going to
completely remove old oil stains) was, first i used bleach and a
pressure sprayer, then i used kitty litter(or equivalent) and crushed
it over the stain to help blend the stain in with the existing concrete