Customer and professional reviews of Marvel Mystery Oil are generally positive, with many reviewers citing improvements in engine noise and overall performance when the product is used as a lubricant or fuel additive. Some users also observe slight improvements in efficiency when using Marvel Mystery Oil.
Marvel Mystery Oil was named by its inventor, Burt Pierce, who claimed that the product's composition was a mystery when people asked about its ingredients. Several reviewers believe the product is primarily composed of paint thinners such as naptha, with a small percentage of dichlorobenzene. The presence of dichlorobenzene is thought to account for some of the beneficial effects of Marvel Mystery Oil, as it acts as a sludge remover. However, the chemicals composing this product also cause motor oil to become thinner, accounting for the common complaint of many Marvel Mystery Oil reviewers that the oil in vehicles treated with the additive does not last as long as it normally should between changes.
Many reviews of Marvel Mystery Oil claim that the engines of cars treated with the additive run more smoothly, with some consumer reviews crediting it with fixing problems such as persistent Check Engine lights. Technology publication Popular Mechanics reports that Marvel Mystery Oil is useful for quieting engines and restoring seized engine components. Some reviews report small gains in fuel efficiency when using Marvel Mystery Oil, though these gains are typically only a mile or two of additional travel per gallon.