You can find umpteen threads on this issue in the recent past. Basically I think most people use either Lionel Premium Fluid or Megasmoke. I have not found any great difference in smoke output between them but have stuck with Lionel's own product for recent Legacy engines where I find the smoke units more finicky/less output than with older models. Lionel Premium is obviously a different formulation than others because it does dissipate/evaporate in the container over time whereas neither Megasmoke or indeed MTH smoke fluid do, at least in my experience.
Possibly related point: A long while back Mike Reagan commented on this Forum that Lionel Premium does not leave the same residue on the smoke unit wicking that Megasteam does. I have not seen any great difference in the two in this respect but someone whose engines get more run time than mine might be able to comment.
