Dishwashing detergent, for the most part, is safe for car finishes. Nothing in dish washing detergent will actually harm your vehicle’s finish, but there are some differences between dish washing detergent and soap that is made specifically for vehicles. One of the differences is that some of the ingredients in dish washing detergent will also effectively strip the waxes and polishes you may have applied in the past off of your vehicle’s paint. While not an issue, care should be taken to re-wax your vehicle after washing it with soap made from dish washing detergent. Beyond this, car wash soap has ingredients that can help it dry with less streaks and spots when compared to dish washing detergent. However, with the right added ingredients, homemade car washing soap made partly with dish washing detergent can be quite comparable to a soap made specifically for automobiles.
For more info, read How to Make Your Own Car Wash Soap