You know, when I left the Big L I thought I would never have to revisit this topic! I guess not.
Okay, here is what my experience has been. JT Mega Steam fluid leaves a tacky residue inside the smoke unit (and all parts the smoke fluid touches; stack, funnel, shell, etc). Te carrier fluid that JT uses leaves a tacky hydrocarbon deposit on the batting and all throughout the smoke system. Does it smoke well? Yes. Does it smoke long? In modern Lionel equipment my experience has been that prematurely wears on the system, causing more frequent repairs to the smoke system.
For anyone wanting to perform this test themselves, it is very easy. Take an AA set (modern Legacy with a track IR LED) and put Lionel Premium Smoke Fluid in the powered A unit and JT in the non powered smoke unit (or vice versa). Then just let them run, you will see which unit degrades faster. Then, when you take them apart and compare the two units you will discover this tacky residue I referred to all the time.
When I was at Lionel we tested and built our products using Lionel Premium Smoke Fluid (which was the old unscented TAS smoke fluid). This fluid is not as heavy as the JT fluid and leaves no residue behind, sans the hydrocarbon on the batting (that dark brown/black "charring" that prevents fluid from making it to the element). If you remember all the videos I did, where I talk about a needle applicator bottle, that allows you to get the fluid down in the smoke unit (in the batting) and not all over the inside of the funnel/stack. Anywhere you have liquid fluid the smoke vapor likes to cling to, a clean stack will produce more smoke!
Anyway, that is where I am coming from. Never wanted to start a brand war, just wanted folks to be able to enjoy their trains, without having to get inside them.
Thanks,
Mike