The greying is probably limescale, possibly from washing when the engine is still warm? I use Koch Chemi's 'FSE' for that. Spray on after I've dried paintwork and rub it dry. I mainly use it for where water spots dry on screens and paintwork, as I have really hard water here and limescale appears with every warm weather wash, where water can start to dry off.
Harley do a post wash engine aerosol spray for their black cylinder heads and cases. That masks it and looks like new again, but can leave a silicon residue.