It is NOT illegal everywhere...that's just what they want you to think. My state and county still use used motor oil for dust control on hard packed roads (unimproved). On all but the sandiest soils it will decomposition stabilize at around 3-4". The county hydrologist maintains water testing stations at several locations and the results are published in the paper monthly...he has yet to find a higher than what is expected hydrocarbon PPM in any water samples. Test results typically show the expected PPM from normal highway runoff (grease, tar, asphalt petroleum distillates, gasoline and diesel). By the way, we have one of the cleanest freshwater lakes in the country in the watershed. Not saying it's right or wrong, just that it is legal in some places and is cheap and effective.