Regarding "Running a lower RPMs without load will shorten the life of the engine unless it is pushed up at least once per hour for about 5 minutes. This is outlinede in the Yanmar operation manual."
This is not to challenge the veracity of your statement but I cannot find that in my Yanmar manual. Maybe you have a different manual and it would be helpful (really) to have you quote what it says.
What my 4JH3-HTE Operation Manual says (on page 30 section 4.6) is:
"When operating the engine at low speed ffor long periods of time, race the engine once every 2 hours"..."Racing the Engine, Morse Remote Control Handle Pull out the handle lever...and shift the engine speed from high to low several times...Racing the engine removes carbon built up in the compustion chamber around the fuel injection valve. Neglecting to race tthe engine will cause the exhaust to turn black and lower the efficiency of the engine".
In other words it says to gun the engine several times while it is in neutral.
Like others, we vary my speed between 1600 and 2200 for cruising and can cruise up to 2400 rpm. We also run it at say 2800 for 5 minutes or so periodically. However, I cannot find where Yanmar recommends this. There are postings in this group (not yours) that say to run it full out for 5 minutes, even full out fo 30 minutes but I don't think that is advisable. Seems that vibration is higher and it might be hard on the transmission and c drive as well. Like most of us we vary the speed and run it harder at times but we also do the "Racing" procedure that is in our manual.
Bob and Suzanne, KAIMI #429
La Maddalena, Sardegna