I just sent my car in to have a 12k mile service (oil change rotate the tires) done on it. The dealer wanted to do a "recommended" oil flush on my car to remove "sludge" from the bottom of the engine. It was going to run an extra 120 for this service so I declined. Is this necessary to do? Ive never had to do it before on other cars that I have owned (one being a 99 rs). Is this something that I can do myself? What do I need?