The manufacturing process used to make spindles is they are forged steel, which uses an oil based heat quenching, then machined, which also uses an oil but a different oil type. The final look is dark on the non-machined surfaces from the forging, and shiny steel on the machined surfaces - axle, tie rod holes and ball joint holes. To reduce rust, wax may be sprayed over everything. Phosphate and oil is about as close as you can get on the non-machined surfaces,
Jim