Depends on the amount of each. One liter of oil is less friendly than one dung, but average car leaks much less oil than a horse produces dung. Several tons of

Depends on the amount of each. One liter of oil is less friendly than one dung, but average car leaks much less oil than a horse produces dung. Several tons of dung in one place is not that good, look at big animal farm runoff. Cars were hailed as solution for horse dung problem.