You could try a 50/50 mix of SImple Green degreaser and water on the hard to get spots. Regular oil will deteriorate rubber bushings. If he used synthetic oil it shouldn’t damage plastic and rubber parts “as much”. You’re still gonna want to clean it up though. Just regular dish detergent that is phosphate-free will work well too to clean the oil and not damage the metal, plastic, or rubber. I agree with the car wash idea. Don’t use too high of a pressure on a commercial pressure washer because you can risk damaging sensors and the factory undercoat. I usually just let the pressure washer spray at its lowest setting without using the trigger. Might have to go under there with some rags or cut up T shirts, then dispose of the oily rags appropriately (ie. at a recycle center or bio hazard waste collection center) . You don’t wanna just leave oily rags laying around. I also like to go back and re-lubricate the vehicle’s bushings and plastic parts with a silicone spray after the cleaning. Hope this helped.