There are two products to consider here. Neither is recommended but for different reasons: Dishwasher detergent contains abrasives to cut through grease and baked-on food. Those same abrasives cut into the clear topcoat of your car's paintwork. You won't notice it at first, but it will eventually dull the surface. Dish soap that you put in a sink of hot water is not usually abrasive. It's better than not washing your car at all, but it can still contain chemicals that can soften and eat through wax coatings, potentially exposing the paintwork to harm. Many people wear rubber gloves when doing the dishes to protect their hands, so it's not a good idea to put that stuff on your car! Dedicated car shampoos are much gentler.