I used MMO in the crankcase* of my Toyota PU (22R) the engine had no real issues, but it is 26 years old, and was a little shaky at Idle and low RPM.
I ARx'ed the engine as per instructions 2 years ago, with no discernible results (I monitored compression, fuel consumption, vibration etc.)
This Fall I decided to try MMO.
I purchased a Quart at Autozone while visiting Port Angeles WA
Half a Liter was added to the oil pan for the last 2K of my normal OCI (oil was it's normal 1/2 liter down at that point)
Suddenly at about 200 mile after adding, Something happened!
The engine, for the first time, ever became smooth* and the Truck (again for the first time) could be driven in 2nd or even 3rd gear without gas at idle speed. I have not checked compression and I have not checked fuel consumption (My driving at this time is too inconsistent) but 'feel' there may possibly be a slight improvement (it's certainly not worse)
Old oil at the end of my OCI was VERY black.
I don't think at this time I will use MMO on a regular basis, but may use it in a similar manor as a 'once a year' clean-up.
* Even the wife noticed this, and in the past she has driven on a flat tire and not noticed!
*at that time MMO was used ONLY in the oil. I have since used it in the gas, but have not had such spectacular results (just seems normal)