Engine oil traps dust and dirt, so it needs to be changed at regular intervals. Good engine oil will ensure your engine will run smoothly and efficiently.
If you notice your engine leaking, it is a cause for concern. Visit a mechanic and get it checked out.
A new filter will help in a smoother flow of clean fuel to the fuel pump and engine and will ensure less build-up inside the engine.
Your check engine light is your car saying it needs to be assessed. Visit a mechanic and get your engine inspected for any transmission or fuel delivery problems.