For what it's worth:
My totally unscientific experience over the last 9 years and nearly 1700 tach hours:
When I added MMO to fuel (at the ratio published on the label):
- I premixed the fuel with the MMO in 5 gal Home Depot Aviation Supply buckets, then poured it into the left fuel tank. 10 gallons, balanced against the similarly-filled right tank with no MMO.
- Next morning, that same left tank was leaking fuel from the fuel sump sample valve. The leaking fuel was pink. I do not know what seal O-ring material was used on that sump valve. I don't know if it was rubber, vinyl, silicon, unobtanium (I know...shame on me).
- My conclusion: MMO causes some sump valve O-rings to soften and allow a fuel leak.
- My corollary conclusion: MMO in fuel is not good in my RV.
- The unknown: I have no idea whether MMO was indeed actually useful or detrimental as a fuel additive for valve sticks, piston health, fuel economy, global warming, world economics...etc.
When I added MMO to my oil (at the ratio published on the MMO jug label):
- I have IN THE PAST added MMO to my 20-50 oil...and then experienced stuck valves (more than once, and always in the wintertime). Sal (the Cylinder Whisperer in north Texas) said: "You added an oil thinner (noun) in the summertime to thin (verb) oil that was already thin (adjective). It was then thinned (verb) even thinner (adverb) by the summer heat."
- Per Sal's strong recommendation: In the summertime I use Phillips 100AD oil and added MMO. He also suggested I pay attention to Mike Busch's red box advice.
- Per Sal's strong recommendation: In the wintertime I use Phillips 20-50 oil with NO added MMO. He also suggested I pay attention to Mike Busch's red box advice.
For three years now, I have had no further problems. Thus:
- My conclusion: MMO in engine oil is good in my RV, when combined with refinements to my CHT management (think Mike Bush) and engine oil type during hot/cold seasons.
- My conclusion: MMO in fuel tanks is not good for my RV.
So here's my battle plan:
- Winter: Philips 20-50, no MMO.
- Summer: Philips 100AD with MMO at each oil change, with MMO at the labeled ratio.
By the way, Blackstone loves my oil analysis numbers.