Can you point me to some threads?
My first thought is that one serves as a better UCL, the other as a better cleaner, but I can't say Ive seen threads on this.
I swap adds every tank or at least every few tanks. I just took a gallon of lucas out of storage a few weeks ago, and I do have to say that something about how the car ran changed once I put it in!
I get the same feel when swapping to MMO though.
FP, schaeffer's, etc. can do certain things, like clean, lube the fuel pump well, etc., but MMO and lucas seem to give a different feel than other adds.