I don't remember my father ever using STP oil products, but he seemed to be a big believer in their gasoline additives. I always considered his actions overkill, since he used them primarily to prevent ice in the fuel system during Wyoming's harsh winters even though the cars he used them on spent most of their lives in a garage. The gas treatment seemed to work well enough on our carbureted cars, but he made the mistake of using it in my Mom's 1985 Honda Accord SE-i (Honda's first fuel-injected car) and I remember that car had a distinct disliking for it It was the only time I remember that car ever protesting, and boy did it protest. Spewing black smoke, seeming to miss on various cylinders, and generally horrible performance until it warmed up. I believe that's the last time he ever used it, thankfully...and I've never used anything STP because of what I witnessed that day.