Marvel's Mystery Oil, anybody use it?

I have an aging lawnmower that I'm considering using it in, at least to clean it out. The mower is 15+ years old with questionable maintenance before I started taking care of it about 8 years ago (yearly oil and air filter change, blade sharpening ).

I also have a friend's lawnmower that receives little to no maintenance that I will be changing the sludgy oil in soon.

Anyone use MMO either as a one time clean-out or as an oil additive in small engines? Benefits? Damage?