Marvel mystery oil:
http://www.audiosears.com/MSDS/FS0433-Marvel%20-%20Marvel%20Mystery%20Oil.pdf
https://en.wikipedia.org/wiki/White_spirit
https://en.wikipedia.org/wiki/Tricresyl_phosphate
https://en.wikipedia.org/wiki/1,2-Dichlorobenzene
https://en.wikipedia.org/wiki/1,4-Dichlorobenzene
I'm not a chemist, but i'm not shure if Dichlorobenzene might affect rubber sealings (IMSB).
In general (even cheap) engine oils contain enough detergents and rust inhibitors. Personally i don't see why Marvel Mystery Oil should have a positive effect in this case. If you have corrosion in the engine i owuld recommend to get it warmed up completely and flush the engine oil to get all particles out. If parts have been rusted they have an inreversable structural defect. So at the moment i don't see why something like Marvel Mystery Oil should have a positive effect.
Have a question as a non US residential: are there any given time limits by the US government to get a rebuilt title for a car?
Regards, Markus