As you may recall the U S military once rostered a number of FM's, particularly H10-44's. In 1980 the local Amtrak rep arranged a tour of the military's locomotive rebuild facility (it's still there) at Hill Air Force Base, Utah. We had discussion with the staff, and topic of FM's came up. One was being rebuilt at the time. One mechanic made the observation FM's had to be under heavy load to keep from carboning up. Few military assignments required really heavy loads, so he believed FM's were not an ideal military unit. To "clean out" the OP cylinders of FM's he said (I kid you not) they would run Dutch cleanser in the cylinders with the engine running.