My in laws use 5-20 mobil 1 in their 2003. 252k mi. Chain rattles like crazy on start, consumes quite a bit of oil....most I've personally dealt with when borrowing it a couple years ago was a quart and a half in about 700mi. Highway drive, not towing. Loaded decent with people and gear but under payload for sure. My 03 has had a similar life as to theirs, was bought in and operated in the same state as theirs, had similar drive characteristics from previous owners as theirs (mother in law is in real estate and cared people around the city...probably 70% hwy) and my previous owner commuted from about 25mi west of indy, again hwy for work for a decade. Only significant difference is the in laws have towed their small boat with it a dozen or so times and what oil was used. Previous owner of mine used 10w-30 for about 100k before I got it. In laws 5w-20 since day 1.
No 2 motors are identical but we literally have the same truck, down to the year and almost all options except I dont have advance track and they do. I have LSD, while theirs is open but that's the extent of the differences. Both had regular service throughout, mine a bit more in the hard parts...wheel bearings, calipers, suspension bits, etc. But motor wise no major work has been done to either beyond preventative maintenance and consumables like plugs and coils.
My 03 is whisper quiet and buttery smooth compared to theirs. My mother in law has a hard time believing it's the same vehicle and is annoyed theirs deteriorated as so. Being I have as much data on both vehicles (I have all service records since new on mine, theirs was owned by my wifes grandfather since new and was handed down when he passed a few years later) there arent too many variables other than oil choice and personal driving habits.
Mileage wise I've not taken a hit, in fact this trip to Tennessee yielded the best hwy mpg to date. Granted it had a new fuel filter, air filter, oil, tires rotated and inflated properly, etc. I managed 17.4, calculated by actual fuel consumption when I filled back up for an 85% hwy drive. The off hwy were curvy state roads but no major slow downs...probably averaged 40mph+ at any time on those. That was with 2 adults 2 kids w 30ish lbs ea car seats, 2 dogs and about 250lbs of stuff. My plugs have about 40k on them, coils well over 100k.