Challenger smoke fluid is original Gilbert scent and smokes well. It is the right viscosity for New Lionel AF smoke units. Original Supersmoke works well in Gilbert engines but is too thick for best results in the new Lionel fan driven units.
JT's Megasteam seems to be the preferred smoke fluid for O gauge trains. They make a cedar scent but to my nose it is not quite the correct Gilbert scent. It works fine.