We ran the Class 8 diesels 24/7 in my last job, only shut down to fuel (about 20-minutes, about every other day).
At night, due to the heat, we ran the AC to keep the sleeper cool. Advice from our company shop was to run the High Idle feature to keep the oil pressure up, and the chance of wiping the bearings waaay down.
Due to health, I have to idle my DODGE Ram (V8-318) for long periods in the summer, when at a stop. I use the right foot to kick the rpms up to about 1,100 to keep oil pressure above 40 psi.
Daimler-Chrysler used to fit police-spec vehicles (from the 1970's forward) with High Idle controls for the same purpose: Keep alternator at cut-in point, keep oil pressure up, and keep engine cooling fan at higher speed to overcome the lack of airflow through compartment.
For a comparison, I normally average 22-24 mph for each tank of fuel (per elapsed time meter), and in summer this can drop to 11-12 mph. Fuel mileage drops from around 13+ to the low 11's.
This kind of hard-use causes me to select premium synthetic oil and synthetic media filter, plus the use of LC and FP (with ARX clean-ups per spec). Having until 2004 only owned non-computer-controlled cars, I can attest to fuel dilution in carb'd cars as being significant, and, though modern one's light-years better, still, I want to see little or no degradation over time and miles.