I remember in the "old" days (1980's-1990s), it was the *SCHNIZZLE* for keeping pistons and piston ring grooves clean in aircraft engines (when added to the fuel).
These engines run on AvGas, which has high lead content.
MMO had/has:
tricresyl phosphate (an antiwear and extreme pressure additive, 0.1-1.0%), and
ortho dichlorobenzene (a softening and removing agent for carbon-based contamination on metal surfaces, 0.1-1.0%)
and it really worked great for helping keep aircraft piston engines clean internally.
My BIG questions is whether the formula has changed (gotten weaker) over the years or not......