I had to put it up here because it's surprising that a lot of people doesn't know that not replacing the oil soon enough is almost as bad as not using oil at all.

Most of the time, they just top it up with oil believing that for as long they get the level right, the engine is fine. Little do they know that there are tiny oil passageways and orifices inside the engine that can get clogged up with sludge. That becomes a recipe for oil starvation which can eventually kill the engine.