I'm not too sure about the "mechanic in a can" products but Lucas does make some good additives. Saved my Camaros engine when the oil pump pickup took a day off at the track one day
You really could actually feel it on the internal parts when I tore it down.
Are you sure its not spark knock? Spark knock will sound like a few marbles in a coffee can but only when accelerating. If a shot of premium gas fixes it then thats the problem. Its usually caused by overadvanced timing, poor gas or compression being too hight for the octane used.
Lifter tick is constant "tapping" but usually
goes away after the truck is warmed up. Injectors can make a very similiar sound & may come & go without warning. If you gas mileage took a dump I'd be looking at the injectors.
Rod knock is the heavy thud sound that only comes on during acceleration (up a hill). I'm thinking its not that at all, but maybe more of the injector or lifter tick.
I'm not sure if the computer can limit total timing, but it may be worth a look at with the timing light.
Did you try the ole vacuume hose in the ear trick to listen to different areas? Its a old trick but works pretty good for finding a loud injector!
FYI, I'm running a 180* thermostat & have had no new noises (just the same old ones!). I think the worst thing the 180* stat would cause is a rich fuel mixture & little heat on cold days. Temp stays just below 200* all the time according to the factory gauge.