Well dont worry, this is a common problem, don't try to clean them while washing. First wash your car get rid of mud and dust particles. Clean it with soft chamois leather. Let the car dry, there should not be any water left anywhere on the body. If you have the carnabua wax, apply on the parts where you find the tar marks, these are not only tar marks, these are oil deposits also. Apply wax in circular motion with the sponge a thin layer, let it dry, it will form a haze. Now there is some physical effort required, use a soft cloth (your used nightwear or undergarments can do), try to rub on the wax applied turn the cloth side as you rubbing. Most of the tar mark/ oil mark will go off while applying the wax itself, and remaining the tough one will go while rubbing. The result is clean and shining surface. Do not use any detergents, your nails to get rid og those marks it will levae scratches on the surface. Use good quality wax. I use Formula1 carnabua wax. Let me know if you want more information.
I was planning to write in detail about it with pics, will do so soon.