If you have a lot of greasy stains, mold, and mildew, we recommend using a detergent or soap before you pressure wash.  These liquid cleaners will loosen the dirt from the open pores in the concrete and give you a more effective removal of stains and dirt.
Mold, mildew, rust, and oil stains need special treatment and cleaners.  Rust is a chemical stain.  But oil penetrates the pores of the concrete and is difficult to remove.  Pressure washing will remove the surface oil allowing the oil inside to resurface, and the stain can return.