Why do cars need oil changes?

Answer: Oil degrades and gets contaminated

Oil evaporates over time — Wrong. While tiny amounts of oil can evaporate at high temperatures, this isn't why oil changes are needed. The main reasons are: oil breaks down chemically from heat and oxidation (losing lubrication effectiveness), and oil gets contaminated with metal particles, combustion byproducts, and dirt. Degraded, dirty oil can't properly protect engine parts from wear.

Engines consume oil as fuel — Wrong. Engines don't burn oil as fuel (ideally—if they do, it indicates a problem like worn piston rings). Engines burn gasoline or diesel. Oil's job is lubrication, cooling, and cleaning. Oil changes are needed because oil degrades chemically over time and use, and gets contaminated with combustion byproducts and metal wear particles, reducing its protective effectiveness.

Oil degrades and gets contaminated ✓ — Correct! Oil degrades through two processes: (1) chemical breakdown from heat, oxygen, and pressure, reducing lubrication effectiveness, and (2) contamination with combustion byproducts (soot, acids), metal wear particles, and dirt. Degraded, contaminated oil can't properly lubricate, cool, or protect engine parts, causing accelerated wear and potential damage. Fresh oil restores full protection.