best for the engine

Operating the engine in it's sweet spot will get your your best MPG and that would be the best thing for the engine. Things like letting the engine cool 3-5 minutes after running is good.

 

The boost and RPM work the same. Lower the RPM, lower the boost. Higher RPM, higher boost.

You need to find the engine's sweet spot; the place that gets you the most MPG at cruising speed. It will vary from engine make to engine make.

The faster you go, the higher the RPM and boost, obviously. Most newer engines such as the Detroit DD-15 well have a sweet spot of anywhere from 55 MPH to 65 mph, typically in the 1200-1300 RPM range. A lot will be determined by the rear axle's ratio. The higher the number, the slower you will have to go in order to maximize fuel economy.

It'll take some trial and error to figure that out tho. A huge help would be if you have a display that shows your current MPG while driving and you can adjust accordingly.
Another option is to have a 3rd party device, such as a Scan Gauge to help you see your engine stats.

Don't be too hasty in determining your results in a 100 mile stretch. There are numerous variables that will affect the results: wind, weight, terrain, etc.

Just because you got 1 result for a small stretch doesn't necessarily mean that's your sweet spot. Trying to determine the sweet spot for your individual truck takes time....at least 1 month of varying speeds over varied trial terrain and weather conditions.

 

The least amount of boost will give the best economy. Until you start climbing a hill. When on hill you are almost full or full throttle and going slow. Pay no attention to the instant read out. Here's an example if you pull that hill super slow with low boost and low rpms it may read better on your instant read out but you're on the hill longer. Down shift get your rpms up to max boost and your instant read out will show using more fuel but you will have a shorter burn time. Make sense?

 

You will get the best fuel economy with the lowest average boost psi. When climbing a hill you have to pick a place between keeping boost very low or full power. I always found keeping it under 10 psi when open road and letting the speed vary a little bit combined with hill climbs keeping it 20 or 25 psi while climbing worked the best. You don't get to the top of the hill very much faster by hammering it all the way up.

 

There isn't a way of knowing what the engine is doing at 10 psi to come to a determination of what's good, unless you have Insite hooked up to it, and duplicate boost every time the same way, with the same weight, same throttle position, same grade, same gear, same everything. Boost pressure with the ISX is tricky, but very important if you know the peak boost according to the calibration file. A example would be if you had a plugging EGR cooler, you can obtain well over 50 PSI on a climb, possibly to the point of blowing a clamp off, I've had it happen. You can also have really low boost due to a stuck open egr valve. On flat land if your at 10 psi, that boost could be from EGR or air, again it's impossible to know unless it's hooked to Insite. So to answer the question of what's better for the motor, simple, pay attention to your MPG with a calculator and change your oil well within Cummins recommendation according to your MPG.

 

Boost and RPM are not indefinitely tied together.
Turbos are variable. You can run 1300 rpm at 20 lbs and 1300 rpm at 50 lbs depending on the load on the engine.
Engines haven't had a true "sweet spot" since ECM's and electronic fuel management.