I added lead to my sons. There was a weight limit in our pine wood derby. We did not exceed that weigh. Most others added weight as well. His car won.
The company that puts together the car kits sell a weight set made out of tungsten. You can also buy a wheel kit where all four wheels came out of the same mold.
I’ve used lead, put “O” rings over tires to cut down on friction, used moly (slicker than teflon), etc. Painted cars with DuPont Chromalusion ($$$$$), ghosting in flames, etc. I loved building pinewood derby cars - always legal though.
