I add MMO to my gas tank every time I fill up, 4 ounces to each 10 gallons. I have been adding MMO to my gas tank at every fuel fill for a little over two years now and I can tell you that yes my truck does get very good benefits from the upper cylinder lubricant properties of MMO. It increases compression by improving the piston ring seal, which improves power. MMO in my fuel helps provide for a nice smooth and even idle and it is a very mild lubricant and cleaner for my fuel injectors. MMO also helps to offset some of the negative effects of corn ethanol that is found in gasoline, it helps restore some lubricity to ethanol-laced fuel and it lubes the fuel pump, helps it work more efficiently. A very welcome side effect to using MMO in my fuel is I get about 1.25 to maybe 2 more MPG than I do without MMO.
MMO in the crank case is a different story. I have added it to my oil a few times, only during the last 750 to 1000 miles of an OCI as a cleaning agent. When I drained the oil it was pitch black and the filter was very heavy so I know the MMO had to have done some good cleaning in my engine's innards. MMO in the oil can help quiet noisy lifters or other engine noises. MMO will thin out your oil, maybe not by a full grade, but it will thin it out some and that may account for the increased power and engine pep that some people (myself included) have reported. MMO can also help improve cold starts when used in oil. MMO will also burn off pretty quickly in oil, so if you add it to your oil it is best to keep a close eye on the oil level.
Another BITOG'er here once said, "If you need to add MMO to your oil all the time, you have special problems." I sure do agree with him. I just use a good oil and do not see the need for MMO in it. However, I am currently on the first run of PYB I have ever used in my truck. It is doing fine, truck runs great on it but I might add a pint of MMO at the end of this OCI, just to boost the cleaning properties of the PYB. But unless you have specific engine problems there should be no need to use MMO in the crank case all the time. A good name brand oil and filter should be all that is needed.
So, yeah, I use MMO in my fuel all of the time. I once did an experiment without MMO and my truck did not run the same. I get great results from using it in my fuel. It works great for me, some others have had bad results or no results at all. I was skeptical of it too, you just have to try it for yourself. Just keep in mind, MMO is a great product but it is not a magic cure-all-in-a-bottle. You have to be consistent with it and it may take a few tanks of gas with MMO to see or feel any improvements. I personally believe the best results come from the use of MMO in older, higher mileage engines that need the help with the improved ring seal and compression.