Engine oil is the lifeblood of your engine. It keeps moving metal components like pistons, bearings, and valvetrain parts from rubbing together directly. The oil is pumped from the pan, through a filter, and down channels to form a microscopic hydraulic wedge between parts. It also carries heat away from internal engine surfaces, seals the piston rings, and holds dirt particles in suspension so the filter can trap them.
Oil degrades from heat and blow-by contamination. The additives wear out, and the oil thins out or thickens into a black sludge. When the oil level drops or the oil gets dirty, friction wear increases. This leads to worn bearings, oil leaks, and eventually engine lockups. Checking your dipstick regularly is the easiest way to prevent engine failures.