Ahh the MMO debate. Another timeless classic.
For what it is worth.... After having several mechanics recommend MMO I started mixing some with the fuel on my aircraft and running a quart in the crankcase. The benefits as I see them are:
(1) Longer fuel bladder life (according to a recent Aviation Consumer Article on Fuel Bladders - not oil) oil protects rubber and plastic. This is particularly relevant if someone chooses to run auto fuel (which I don't). Auto gas can have ethanol and other additives that increase octance but which are harsh to old airplane parts not designed for its' use.
(2) Possibly better corrosion protection - depends on the oil you use normally and what additive package is in that oil.
(3) Smoother engine operation and longer valve guide life. In most reciprocating engines high thermal stresses are found at the exhaust valve guide. I am not sure if the smoother operation is due to better oiling of the lifters, or smoother operation of the valves through their guides, or both. But MMO does seem to give you noticably smoother operation.
(4) Less gunk build up on the exhaust valves, seats and guides. Theory is that the oil gets vaporized and keeps the deposits soft where they get blasted away. Today 100LL has more lead than the old 80/87. So this is likely more of a benefit in low compression engines designed for the 80/87 fuel (like mine).
The other side of the coin, although I have no data to back this up, it makes sense:
(1) Higher EGT's. Any oil will burn hotter than AVGas. In my airplane I typically adjust the mixture richer to keep the temperatures where I am used to seeing them in AVGas use. I know there are arguements that we should be operating lean of peak and that these temperatures don't matter, but for me a $35,000 engine installation is not something I'm confident experimenting with. So I burn an extra half a gallon an hour to keep things cooler.
Some have tried to analyze what is in MMO. The product was developed shortly after WWI and at that time phosphorous and zinc were used as "racing" oils. These provided lubrication under extreeme conditions and had cleaning properties as well. These products were phased out with the advent of catalytic converters in cars. Curious consumers have sent MMO off to labs and come back with the following reports (reportably from Blackstone's):
It is a light napthenic oil ( about 3 wt).
About 20% solvent-probably mineral spirits
Dye
790 ppm phosphorous
Wintergreen for smell
The solvent would tend to provide some cleaning ability, at low temperatures.
The low viscosity would tend to act as a detergent. The phosphorous probably provides an EP (extreme pressure) ability, or would tend to prevent scoring or seizure. This agent is already in most motor oils.
At lunch a couple of years back I had a couple of manufacturers' air safety engineers and NTSB IIC's at the table and asked their thoughts on MMO. I was surprised to hear that the tech reps say that they thought running MMO was a good idea for the reasons I outlined above. The NTSB Investigators said they had not seen an accident due to the use of MMO, but they had no opinion at all as to whether it was a good thing, or a bad idea.
So to wrap up - I think MMO is a good idea, particularly in engines designed for 80/87 gas. The newer and higher compression engines may not see as much benefit. In any case you would not want to run more than a couple of ounces per 10 gallons.
As far as the airplane "I Follow Roads" experienced, that sounds more like a really old airplane that had seen a good bit of auto fuel. From what we do know about MMO the properties of the stuff the type of contamination he described does not fit. In fact many farmers religiously use the product for systems that are going into storage as a preservative to keep them clean and reduce maintenance costs.
