• Auto manufacturers recommend against using oil additives; some will actually void your warranty if you use them. The top-selling brands are formulations of the same key ingredients. Nearly all use a simple 50-weight motor oil as their base which already includes all the performance-boosting additives. If you are still interested in buying these additives, it is important to know something about the ingredients.
  
  

PTFE

• Many of the more popular brands rely on polytetrafluoroethylene as the primary active ingredient. Some well-known names trading on this technology include Slick 50, Liquid Ring, Matrix and T-Plus from Kmart. PTFE is actually Teflon, a registered trademark of DuPont, and its inclusion in these products is a source of much consternation for the chemical giant. PTFE is a solid; the additive companies claim that this gives their product its ability to coat the engine's moving parts. Unfortunately, as a solid it also coats the engine's non-moving parts, like oil filters and inlets, where it can build up and act like a dam.
  
  

Zinc

• Zinc dialkyldithiophosphate, shortened to Zinc-D, is an additive already included in almost every manufactured engine oil available on the market. Products such as STP, Mechanic's Brand and Kmart Super claim the superiority of zinc products over PTFE-based brands. The problem with zinc is that, since it is already in your engine oil, too much can actually build up on valves and foul spark plugs. Oil manufacturers have actually decreased the amount of zinc in their products in recent years as it was implicated in premature catalytic converter failure. Zinc only works when there is metal-on-metal contact, which, under normal conditions, should never happen. If you drive a race car or you like to ride the red line in your Honda Civic, maybe additional zinc would be helpful.
  
  

Detergents and Solvents

• Older brands like Bardahl, Rislone and Marvel Mystery Oil do not promise miracle protection for your car's engine. They simply claim to dissolve deposits and keep your engine clean, which they do with a fair degree of success. The most common ingredient is kerosene, but some brands use a combination of other solvents like xylene, acetone and isopropanol. Cleansers are effective on older or abused engines, but the problem is, they can work too well. Too much will dissolve those built-up carbon deposits and keep going right through the protective layer of engine oil.