a good 2500psi power washer with a variable tip wand will do the job for ya. You can rent them from Home Depot or Sherwin Williams. I'm sure there's other places too but those two are the only ones I have experience with. You could also buy a decent power washer for ~$300. I've power washed many homes and buildings and have not needed to use a cleaning solution. If I was going to simple green would be my first choice. Spray it on, let it sit for a couple minutes them blast it off. If you do choose to use something like simple green to clean it, do a test spot somewhere not noticeable that way if the solution fades or streaks the siding it won't be an eyesore. Most likely you won't need the solution.