The simple reality is motor oil is comprised of a multitude of long organic chains. It isn't so simple as using a store purchased chemical to remove it since oil companies dispose of various of their oil and oil like organic compounds in motor oil. It is a sort of perfect dumping ground for a mix of chemicals. I know this for a fact because I was an oil and gas industry specialist for a part of my life.
The old mechanics method is the best. Sulfuric acid, watch and then wash down with loads of water to make sure it is diluted enough to not hurt anything. I can hear the environmentalists going nuts, but it happens all the time in the real world. We are subjected to very questionable chemicals. Sulfuric acid is a relatively as chemicals go since it emulsifies with water very nicely, and since water is a base, it doesn't take a whole lot to weaken it to the point it won't hurt animal or plant. Be careful though, sulfuric acid is a different animal than hydrochloric acid or anything in your kitchen, most soaps are alkaline, as are most drain cleaners. The few acid items in a kitchen don't even start to measure in strength to sulfuric acid from say a battery refill canister. It splashes easily so pour down bent over, buy a cheap mop to wipe over the area you wish to clean, and then dunk it in a big bucket of fresh water when finished. That will dilute it enough to rinse the first go at the surface, and keep filling the bucket and dunk the mop once each time so as to keep it getting more dilute also.
Leave the acid down long enough for the stains to be about 75% gone. Completely cleaning the stains will make your concrete floor so perfectly white, it will look like the day the concrete was first poured. That is like a 70 year old movie star wearing A1 white dentures. Just don't look right. But your stains will be far cleaner than you will get with any other method. If you want them completely clean, leave it down longer, but it might look bit silly.
That is my offering on oil stains. Please wear hand, and eye protection. And, officially, I am saying loudly that this can be dangerous, and don't recommend it. So don't blame me if you burn a finger off. It works though.
