Engines will gradually eat up engine oil. It's what happens, when it's been in use for a while, but if it's old, it's probably not doing its job quite as effectively. And when it's not effectively doing its job, the engine is going to use more of it to make sure it's still running smoothly. When you see the level getting extremely low, don't just fill it back up. Check the consistency. It might be time to change it out.