The Americans on the tractor forums swear by it. I've never seen MMO in Australia, and I had to query what they were actually talking about, when the subject of MMO was brought up.
I have a sneaking suspicion that there's little difference between Redex upper-cylinder lubricant, and MMO.
Then again, automatic-transmission fluid looks very similar, too, and is reputed to work wonders in old diesels, when a quart of ATF is added to the diesel tank.
It's supposed to very effectively clean out gums and varnishes, unstick stuck injection pump plungers, and generally act as a complete diesel fuel system coditioner.