Getting regular oil changes is very important. Over time oil breaks down, heat being the biggest contributing factor to the breakdown. Also there are additives that are put in engine oil that break down and lose their effectiveness over time. There is also the issue of contamination. Small bits of dust, dirt and debris get into your engine and make their way into the oil. All these little bits make the oil much less slippery and much less effective at cushioning the moving parts of the engine.