Be very careful with the pressure washer idea any good pressure washer will detroy concrete in a hurry if you use any tip other than a very gentle one. Even a medium tip on a cheapo home depot style(2500 psi) pressure washer will blow holes in concrete garage floors and sidewalks. :thumbsup:
Plain blue Dawn dish soap. Cover the area in soap and let it sit, spray it off or let the rain do it. As long as you dont wait too long, it'll work great. I've never had oil left behind doing it this way and it's better for your grass than gasoline.
There is this stuff I've used before (the name eludes me at the moment) that you pour on and let sit for for few mins. It soaks up the oil from the concrete and dries into a powder that you can sweep up. AHHH its called "pour n restore" It works reallllyyy well. A bit expensive though.
If you've already cleaned it up and it left a mess, just put down some more oil and spread a shovel of dirt or fine sand over the top. This will draw the soaked in oil out of the concrete. Then use a regular garden hose. No High PSI needed. You may even notice that spot being cleaner than the rest of the area. Good luck!
I work at a concrete accessories store, and there are a few specialized chemicals that will work well for you. They are definately not cheap though. One is just a superconcentrated citrus cleaner, and the other is a soy bean cleaner. Try those if the other suggestions dont pan out for you. (You should be able to find it at you local concrete supplies store). As far as the muratic acid goes, you will want to water it down like 15:1, or it will take off the top layer of you pad i moments. Hope this helps!