There are additives that can improve performance of an older engine, or clean a neglected one. Remember that a lack of performance is an indication of work that needs to be performed, a bottle of goo will not permanently solve any of your vehicles issues. You can, however slow down the break down of the oil you do buy with stabilizers, these work well, and always change the oil on schedule, sooner if possible, it is your cars life blood.

Research new technologies thoroughly before you buy. Make sure you know what is in the product. If they list a secret formula, but don't tell you what it is, be cautious. They may contain harmful chemicals, such as chlorine or detergents. Others may contain PTFEs and metals, which can show temporary improvements, but can ultimately clog filters and damage your engine. Check their MSDS, certifications and independent studies by accredited labs, with their company name clearly listed. Also be sure they have product liability insurance and a warranty. Most brand name oils contain oil additives in them already. Both oil companies and independent researchers are constantly researching new technologies to improve engine performance.