In the crankcase its a solvent that can break up sludge and carbon deposits. Its about a 5 grade oil, so it'll thin out anything you add it to. How much depends on how big your sump is and how much you add. Marvel states you can use up to 25%.
Dosing the crankcase with it is not like dosing the gas. Marvel states its safe to use with fuel and provides dosing information. In the crankcase, they provide only a recommended range and don't make the same statement regarding it being safe to use.
Any solvent added to the crankcase has the potential to cause damage. That doesn't mean it will cause damage, but its something to be aware of before it goes in. I'm running a 10% dose, over a yet to be decided interval, in a sludged engine. I'm not worried about it causing any damage. YMMV,
-Spyder