marvel's mystery oil can be ran in either fuel or oil. in fact, it is great for diesel engines, since most of the sulfur has been removed from the diesel fuel. it does work as a friction modifier in engine oil, but samcr750 makes a good point.
it does work great as a penetrating oil, so if you have a stuck motor or valves, pouring some in and letting it sit for a few days it will likely come free.
marvel may also be known as a blending agent. there is another product from hydrotex called essentialube that is along the same lines. if you put a screw driver, or other metal object in a shallow container the essentialube will actually wick up the screw driver. pretty amazing product. like marvel it has too many uses to list.
aorta got onto another tangent there, but i hope it helps someone eventually.
later,
ryan