I use it in my aircraft, fuel only. It's not a mystery at all. Basically it's kerosene ,with a couple of other things and uses winter green for the smell. No question it provides uppper cyl lube especially in the valve guide area. Many people flying the small continental engines use it in the fuel to help prevent valve sticking, especially as we are stuck with 100 ll fuel. Havn't use it in the Harley as I havn' t found the need , but it shouldn't hurt anything. Don't recommend it on a high time engine as it does soften carbon and remove gum in the fuel system. Not always good if it loosens too much at once. Fuel system is the only place I would recommend it's use, not the oil system, especially with a high mileage engine.
Ron

 

I use a bit mixed with toluene for home made fuel additive.

 

Cool, lube and octain all at once
Ron

 

My dad used to swear by MMO. It would cure a stuck lifter in a heartbeat. I've seen that myself.

It is a good fuel system lubricant. I'm convinced its occasional use in my 07 Sportster was why I never had the fuel pump problems that plague 07 XLs and is the subject of at least one TSB. I put a little in the tank of my FXDC every now and then.

Here's Dad back in the 50s on his Harley. Wouldn't be surprised if there was MMO in the fuel tank or oil tank or both when that pic was taken.

 

I periodically pour half a can in my gas tank on my 92 jeep cherokee,and add the other half to my to my oil , i have 315 thousand miles on the jeep now, original electric fuel pump, original everything in the motor, and original fuel injectors etc. I know for a fact it quiets down a noisy lifter.I know after this many miles it certainly has not hurt anything, I change my oil every 5 thousand miles for what its worth.Use nothing but 30wt valvoline dino oil, and 10w 30 in the winter time.