We have used Terra Wash with GREAT success on this exact type of staining. You will have to use it strong. Id start with a 2 to 1 ratio. one part Terra Wash and two parts water to make a three gallon total solution. Then scale that ratio for the tank size you have. At this mix ratio you will likely need to brush the surface some but many areas will clean without brushing. For a non-brush clean go to a one to one "1/1" ratio or even straight. I know personally that a 1/1 ratio seems to get most any oily residue moving off the surface. Now you may need a little bit of bleach in there to get any organics off the surface as well. If you have to go super strong on the surface let us know and we can get you some bulk discounts on the Terra Wash.
You can also try OneRestore from EaCoChem. It is a awesome restoration cleaner. Use the TerraWash first if you can, I think it will be cheaper. I usually price this as "restoration cleaning" as opposed to "maintenance cleaning." Restoration= more $$.
Also, check your storm water ordinances, in cities like that, there is usually no grass or permeable surface to bio-remediate the wash water. If its running in to a storm drain, and it has any chemical like smell, suds, or appears dirty or oily, city waste water people tend to freak out.
__________________
Chad A. Eneix, President, Water Dragon Inc. chad@waterdragoncleaning.com
Also, big difference between OneRestore is you need to go full "hazmat" if spraying that ona wide scale basis. it's highly toxic to landscape, fumes are bad and it will light your skin up when it lands on you. It's a good product, and does exactly what's it says, but it's VERY HARSH.
I like the Terrawash because it is nice and mild. Does a great job on inorganic stai g like brake dust, car exhaust, etc when sprayed and slightly agitated with a brush.
__________________
Spray Wash Exterior Cleaning, LLC.
Cleaning North Florida, South Georgia, and Tallahassee!!!
Soft washing, pressure cleaning, and Safe roof cleaning!