Supplemental additives are not needed and are not recommended. In most cases they dilute the additives already present and reduce the quality of the motor oil. The normal change intervals in the owner's manuals are not conservative today. Engines have become more demanding of the oil, but recommended intervals have not shortened. We advise against exceeding OEM engine oil drain interval recommendations for your type of service.
Motor oils made with certain base stocks are better than others.
High quality mineral oils can be produced from petroleum feedstocks by a number of different refining processes. Motor oil manufacturers carefully select additives that will provide the desired performance. Engine oil formulations must complete rigorous testing to obtain an API license. I never see fuel savings. The American Petroleum Institute has set specific fuel economy standards for engine oils.