Oil keeps everything lubricated, cool and clean - which is why it gets dirty.
When you start your vehicle, oil begins lubricating your engine. Numerous additives within the oil help it cool and reduce friction between internal moving parts. Oil also cleans away dirt and contaminants, preventing premature engine wear.
For example, for every gallon of gasoline that is consumed, about one gallon of water is generated. Most of this water turns to steam and exits through the tailpipe (which you may notice on cold mornings when that water actually drips from the tailpipe). Some of this moisture does get into the dead air space of your engine. There, it mixes with other chemicals and forms sulfuric and nitric acid. Dirt and residual fuel will also find their way into this dead air space. The motor oil must continually keep this area clean so it sweeps away the dirt and absorbs the acids and fuel.
As a result of all the dirt and contaminants it picks up, motor oil needs to be changed regularly. Also, when motor oil becomes old and dirty, you're no longer getting the full benefit of the additives it contains. The motor oil becomes less effective at cooling and lubricating your engine. What's worse, the increased friction and chemical buildup in used motoKnock, Knock. Wrong Octane!