While I worked on small cessnas at our local hanger (small engine repair+ electrical works) many moons ago: ole tymers worked on old radial Lycomings (still on leaded AV gas though) and a few other oldies , owners swore by mMos: from gas mix to oil. They claimed that it's been used successfully since WW-II with great success so why-not?
They have a point.
However, things aren't this way with newer engines such as Rotax I frequently worked on, which some of them would have a reduction gearbox so that the propeller directly coupled to the crankshaft. This gearbox typically requires something with lots of EP/anti-wear additives inside. My GF's flight buddy switched to using straight-weight SAE 30 with MMO to further thins out the oil and the gears became worn after approx 300hrs of useage.
So, it took a bit of hard convincing to get that buddy to eliminate the addition of MMO in the gearbox (still the same SAE30 from the same oil brand/supplier), got approx. 800hrs out of it before the gear set gets replaced.
Next, we tried substituting with hypoid gear oil with loads of EP+Anti-wear additives (I believe it was GL-4, just run-of-da-mill OTC stuff), and the gearbox lasted over 1400hrs and the clearances still within spec.
Personally, I do not believe in MMO or adding stoddard solvent into anything I work on, not even gas (carb/ported EFI/DFI), most certainly not in engine oil. I never believe in the "so-called" UCL properties, citing the high-mileage 4-cyl internal combustion engines I worked on in the past (with proper maintenance, regular motor oil, etc.), the wear patterns RE: cylinder bore taper, cross-hatch marks, etc. are within factory specs even without them so why bother?
The only OTC stuff that has it's place in all the engines/vehicles that I worked on so far are: (1) B&S fuel stabiliser for small engines; (2 )air-throttle cleaner spray; (3 ) AutoRx for conditional remedy on certain mechanical problems (very focused, not just for that "feel-good" sensation commonly found on joes obsessed with OTC additives); LC20 on small engines motor oil; (4 ) PEA-based fuel injector cleaners for occasional runs in ported EFI cars.
All other mechanical issues, IMHO, require careful assessment to determine what the problem is/are, and proper fix is to be prescribed to resolve the problems properly. No OTC additives would needed in this case.
My 2c's worth.
Q.
