I buy what irritates my nose the least - Megasteam baking bread and Christmas Bayberry are good, and I'm currently running unscented. I haven't seen the gunk Mike R. warned about when I open up a steam generator for a tune up, but an old bottle of scented Megasteam on the shelf shows separation of clear fluid and colored fluid (the scent?). Will have to try Lionel Premium if I ever find it at a LHS.
Smoke fluid is basically mineral oil. I would expect the train mfgs. to test the oil's flash point and use what is safe for use in their smoke units. I recall that Mike at MTH told us to use mineral oil up until they were bottling their own brand, at which point, the generic mineral oil was no good. Lionel's Mike R. recommended using only the Lionel branded oil once they started shipping bottles of Premium.
Other than scents and other additives, is there much difference among the various brands? I would love to see a analysis of the different brands using a standard smoke unit measuring the temperature and resistance of the heating element, along with the volume of smoke produced, viscosity, flash points, and other chemical attributes of the oil.
