Most people use something called TSP or tri sodium phosphate. It is a
type of detergent or surfactant that does not foam much. Be careful
with pressure washers on wood - you can destroy your wood. Best to wet
the wood down with a hose, then spray on TSP solution and then get a
long handled bristle brush and scrub. Some dirt will stay on the
siding even if you spray with enough pressure to dig holes in the wood.
Brushing the wet area with a long handled bristle brush is important -
then power spray it off.

If you don't belive me about the brush - try doing a section without
brushing.