I, too, have a similar story. I used to run GTX in my vehicles and go-carts, as did my friends, way back when simply because it was the least expensive and synthetics were still fairly new.
Even without the "snake-oil" added, it still earned a very respectful score. Which helps answer why our engines ran a little better and a little longer than our competitors back then.
All this time I just thought I was better at feeling the torque when building then the other guy was!
Now, back to that snake-oil. I've been a skeptic too on most of those - because I've tried a lot of them just to see what they'd do.
But with this Prolong additive, it's the real deal IF used with the right oil!
I tried it with the exact GTX listed in his blog in my lawnmower first. Before, it ran very well using Royal Purple 15W-40, consistently took 2 pulls to start, only had an occasional miss while running, and had a slightly audible knocking of the main rod bearing (7 years old in 2016)
After changing the oil to GTX with Prolong it immediately ran just a tad better: It would start half the time on one pull (fully cold from sitting for a week), had fewer missed ignitions, and I could no longer hear the knocking!
A few months later a friend of mine came by and happened to mention he'd just changed his oil. He said he used the GTX 5W-30. (Because it was the cheapest)
He was driving a very high mileage 2000 Ford Ranger (small 2 door pickup truck, inline 6 cylinder engine) with lightly audible valve tappets and heavy rod knocking during start-up or under heavy load.
I poured in a bottle of the Prolong and told him to let me know what differences he noticed.
He called back a week later. Starts easier, quieter valve tappets, less rod knocking, and he said it was easier to pass cars on the freeway.
So then I put Prolong in my three air compressors (using the same straight 30W oil I already had been using in them, not the GTX). Same story: quieter, smoother sounding, and a few degrees cooler.
Next, I put GTX\Prolong in my then daily driver: 1998 Honda Accord EX 4 cylinder with 248K miles. Again, same story: quieter, smoother, easier to start, and perceptibly quicker.
After getting the S500, I decided to use GTX\Prolong as the flushing oil to clean the engine with, meaning I ran it for 1000 miles and then changed it again back to Mobil 1 0W-40. (Even though the dealer had already just changed the oil before listing it for sale)
I can't say I noticed anything different between the two, but given the GTX was a thinner oil I was expecting the engine to be a little louder when listening to it with the hood up or using a stethoscope. It wasn't. But the oil sure was BLACK when I drained the GTX compared to the oil the dealer had put in. It's possible the thinner oil allowed more combustion carbon past the piston rings. Or maybe it truly did do a better job cleaning the internals.
One thing is for sure, my old engines were always very clean when I used to run GTX in them!
I have photos of the passenger's side valve train when I changed the cover gasket before running the GTX.
I'll be changing the other valve cover gasket - possibly this evening - and will get photos as well.
I've only driven 2K miles since putting the Mobil 1 back in, and may be able to tell if the GTX flush made any difference.