yes, it was.
Never did a blind test, of course, but I’d say there’s a slight difference with Mello Yello vs. Mountain Dew, less than any of the other substitutes. Pibb tastes more different than Dr. Pepper, but is still pretty similar.
I don’t think you could tell a difference in either if you let some ice melt a bit in it, like by the time you get towards the bottom of the glass in a restaurant.
Also, Mountain Dew has a reputation (at least in this area) of being a high caffeine drink, though a quick search is telling me that Mello Yellow is high in caffeine as well.
How have we gotten this far without mentioning Sun Drop? It’s hard to find in this area, but it shows up in the coolers at work functions every now and then.