But, you will be surprised what soap and water and a brush will do.
On stubborn spots use some industrial strength 409 or something like that. I like to use some scrubbing powder also.
I have seen good results with those scrubbing brushes that add soap to the end of a water hose.
For more quicker and faster cleaning you can use a power washer. Be sure NOT to aim the wand upward under the siding as it will penetrate and start rotting.