2 questions

[Mattman]

Engine being lugged down to 1200rpm is going to be working harder than one spinning at 1500rpm and not lugging. Your truck should have a fuel mileage graph in the dash. It may not be the most accurate but watching it will show when you are getting the lowest mpgs.

Liner issues due to low rpms and high engine load seems to be a problem for these engines. Most recommend at least 1300 to 1400 rpm or more with say over 30 to 40 percent engine load