Engine heat can break down oil additives, producing acids and other substances. Dirt, rust and metal particles from the engine also find their way into oil. Exhaust gases and fluids like antifreeze can leak into oil during engine use. All of these substances increase the toxic nature of oil to humans. The common way contaminants enter the body is through ingestion of oil polluted water and through the breathing of fumes.
Oil is a valuable resource that is wasted when improperly disposed of.
Used oil can be re-refined and used over and over again in combustion engines. Call a few local service stations. Some stations accept DIY used oil. Contact your municipal government. Some communities maintain collection centers or provide curbside pick-up.