This is a very old thread, but i must chime in as there is a possible explanation to the OP's origional question which i feel should be put out there: "Why would i see diminishing returns after using the UCL for 5 tanks or so?"...
Simple answer: It is Likely the engine management system in your vehicle(s) has adjusted the long term fuel trims (over time) to compensate for the extra fuel density or fuel energy provided by the UCL product, likely directly related to the catalyst fuel trim (part of the fuel management programming deep in your pcm). When first used the pcm may see better o2 values right off the hop (engine side) and provide leaner trims, which = more MPG, but as time goes by the catalyst monitor (post 02 sensors) may notice not enough fuel in the cats (cold cats) and gradually richen the mixture back to its previous fuel trim tables. This is the unfortunate down side to Catalyst systems as they are wasteful bandaid systems that actualy use more fuel than just the engine alone would need to operate economically.
Think of a Catalyst like a furnace, you put fuel in, it gets hot, waste gases and heat comes out... If there is not enough fuel, the catalyst target tempurature will drop and the furnace will "go out", so the pcm must keep a certain amount of fuel in the catalyst to keep it "lit" or operating within its designed temperature parameters. How does the pcm add or subtract fuel to the catalyst? it richens or leans out the engines fuel mixture through injector values, this causes the engine to run rich or lean and the spare fuel left in the exhaust is sent straight to the cats. That is fuel that would not be needed if they came up with a better system and eleminated the catalysts.
This effect may not apply to all vehciles, as most vehicles (even within the same brand) often have completely different catalyst systems and fuel management parameters (yes there are several different types of catalyst systems)
Granted i am no scientist but this is the closest explanation i can come up with as a mechanic, provided there was not other factors involved in your experience.
Good Day to All
