To me, the easiest way to gauge whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above,
but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..
These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings..The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less.
(snip)
In closing, I’ll mention that although this topic is brought up quite often and many people preach that you’ve got to use a diesel hard if you want it to last,
I’m still waiting to find one that was rebuilt before its time due to low speed use..Just the opposite seems to be always the norm.
https://www.sbmar.com/articles/low-speed-running-break-in-of-marine-diesels/