Jim's information is obviously taken from Volvo's advice. My advice (and I should have mentioned this) was based on the Yanmar Diesel engine I had in my R-25 (Classic). Yanmar advises "...workshop manual suggests that their engines be run flat out for 5 minutes after two hours of running at low speeds." Suspect that their 'low speeds' refers more to battery charging than to motoring at less than about 75% of maximum revs.
Here's a snippet I had archived for myself some time ago, obtained from another forum.
Without wishing to insult other posters that I recognise are trying to give their best opinions, a lot of rubbish is talked generally about this subject and there is a great deal of "folklore".
There are several pertinent points to consider.
The first is common to any engine, petrol, diesel or indeed any other fuel and that is temperature and time at temperature. When stationary, as many marine diesels are for long periods there is condensation. This condensation is at it's worst when the engine has been running in a damp atmosphere and cools down but it happens all the time. This causes the lubricating oil to become emulsified with water and this water can cause corrosion. The cure is to get the engine good and hot to drive this off as steam. RPM does not matter greatly but the engine needs to be made to work to reach temperature and it needs to be hot for at least an hour or two.
The second is our old friend carbon build up on pistons, combustion chambers and valve stems. This is clearly at it's worst when the engine is run slowly for short periods when the engine never reaches full operating temperature and optimum combustion efficiency. The cure here is to avoid slow running especially for short periods.
The third is indeed bore glazing where, if the engine is only ever run slowly at low temperatures there can be excessive polishing of bores and gumming up of rings.
These conditions are often seen to have the same cause but in fact with modern oils the latter is not so much of a problem if the oil is changed regularly and never allowed to build up sludge due to emulsification (see first para)
What this all means in practice is that running at peak efficiency means adopting the attitude that about 80% throttle is best. At 60% it's probably still OK but less than this, especially for short periods, will cause problems in the long term. More than this probably won't cause harm in moderate bursts but is not going to improve matters and will as you indicate be fuel inefficient. If you do have glazing, cokeing up, or other problems however, a "good hammering" at full revs for an hour or so can often get it up to high temperature, bust the glaze, get rid of moisture and free gummed rings.
