Jim,
I'm a non-believer in additives not recommended by the manufacturer of the engine or coach. I have this silly attitude that says the engineers that design these products are likely to have more knowledge about the needs of those products than anyone else.
That is not to say someone can't invent a better mouse trap, but if a product did offer improvements that they say on the bottle, the manufacturer of the engine or coach would buy the product, slap their label on it, double the price and claim it was necessary.
That ain't happening.