The first thing you need to know is the type of engine oil recommended for your car. Using fully synthetic Mobil 1 engine oil on your 20 year old Saga is not going to make it go like a rocket. It is only going to burn a hole in your wallet. Engine oil has several functions in your engine. Apart from lubricating the moving components in the engine, it also serves to a certain extent as a cooling agent. So therefore, go to your manufacturers manual and look it up. For reference purposes, we'll look at the oil used for my 7 year old Proton Iswara.
I've always used a 20W-50. This denotes the viscosity or in layman terms, the thickness of the oil. The higher the number before the 'W' is, the thicker the oil will be. So, 40W is thicker than 20W. I have always been a Castrol fan since the early 90's but got a little fed-up one year when they went up in price twice. I've switched to Pennzoil and have not looked back since. I've found it to be smoother and quieter on the engine
Now that you have decided on the engine oil, then its time to go down to your friendly neighborhood spare-parts dealer and purchase the oil and oil filter. The next step is to get an old engine oil container and convert it into a basin to catch all the oil that you're draining out like below:-