This is from Rich Kelly the tech person over at Turtle Wax. This answers some common questions as well as misconceptions. HTH
1. Marvel Mystery Oil is not a solvent in the tradional sense of the word. MMO is an oil mainly. It does contain some tradional solvent in order to thin the viscosity of the oil to approximatel a straight weight oil. Also, it is not considered a detergent based oil in that it does not contain high base number sulfonates. MMO will provide cleaning action to the crankcase, but it does so by different chemistry than detergent oils.
2. MMO contains additional friction modifiers to prevent metal-to-metal contact. Therefore, it improves the motor oils friction reducting properties.
3. MMO has been tested with various seal material that is found in most automotve engines and power trains and is compatible with all of them.
4. As stated above, because to the additional friction modifiers and other ingredients in the Marvel Mystery Oil, using it does not degrade the motor oil but enhances it.