MMO
Some tine ago, I helped a friend with a Pietenpol Aircamper powered by a Corvair. He called to find out if I had any idea as to why the engine (that has set for more than a year after a major OH and covered with MMO) would barely turn.
Looked down the plug hole and no apperrent sign of corrosion. The only solutuon was to remove the jugs. Pat decided to take the entire engine apart. Nothing was found, except that all moving parts appeared to move as if it was full of sand. I asked what he coated it with, he said MMO and STP.
Reassembled without the magic brew, and it ran well.
His partner Sam, did the same to his Corvair, and had the same problem turning it. Like Pat, Sam dissambled the entire engine. The pistons barely moved on their wrist pins. (Pins on Corvairs are shrunk fit) He didn't want to pressed then off. Knowing from Pat's engine that there were no signs of corrosion, I suggested that he "cook" them in hot thin oil (10W) they loosened. Sam reassembled and also ran well.
A third incident was with a C175 with an O300. The prop turned as if the engine was full of sand. After much prodding it finally released. Told the owner to runn it and see what happens. It ran fine.
He used MMO in the gas and in the oil as well. We suggested that he stops using it and switch to multigrade oil. So far as we know, it's still running.
Personally , I use the stuff, but not in engines.
FWIW
TT
RV10