Even after replacing all coil packs and replacing the iridium spark plugs I had (not long) to OEM spec Champion copper plugs, my 136k mile 2004 4.7 SLT AWD is STILL sluggish on plain unleaded. Poor throttle response, bad gas mileage, and downshifting more often. Maybe 14.5mpg at 70mph at the best, and maybe 10-11mpg in the city. It seems to really like mid grade, as most of these problems are solved unless I get crappy gas.

Is this just a thing that comes with age of an older 4.7? My dad has a 2011 Ram 4.7 4WD with over 170k miles and you could probably pour water in the gas tank and it wouldn't even bat an eye, it doesn't care what you put in it, and he gets 17-19 highway.

Any possible causes and/or fixes?